One can hardly look at the constraints which bind traditions in general without beginning with the specific constraints of style which define the Western musical canon, namely the "Classical" style of Haydn & Mozart as distended by Beethoven. What is more, one cannot examine the social context of the later 18th century, the context giving rise to this set of stylistic constraints, without turning to economics. Toward that end, I want to cite Fernand Braudel's Civilization & Capitalism, 15th-18th Century, both for its general insights and approach as well as the more specific context in which it places the 18th century. To be extremely simplistic, the 18th century marks the end of the era of the development of modern economics. For our purposes it means that this was the point at which the "culture of consumption" was decisively entrenched, as mediated by the economic dominance of what was soon to be called the middle class. For most of the West, there is also direct economic continuity to this time, reflected in the basest terms by an uninterrupted paper trail of accounting figures. It is also known as the Age of Enlightenment. I will leave that last thought dangling without further comment.
What must be emphasized at this point is the reciprocal relationship between constraint and tradition, as traditions dictate styles for which one must adopt specific rules or constraints to follow. Stated in the converse, constraints allow listeners to build expectations as a subtext to the actual sonic events. They allow a shorthand interaction between musician and audience, as the expectations built upon constraints can simply be subsumed into the line of argument. The cognitive essence of the piece becomes a sort of "delta" or difference between those concrete expectations and what actually occurs. Specific constraints are nurtured most directly by small, but non-negligible, deltas. This allows some sense of novelty, as well as an opportunity to develop a style, but remains comfortable for the listener. One nagging issue is that this delta, or the "real" content of the piece, is indeed small. It comes immersed in an increasingly large substratum of cognitively empty constraint-validating conventions (we might say the same about the world wide web). While this fact is a source of comfort for some, it is an increasingly tedious fact for others. Since one might imagine that creative people in general tend to fall into the latter camp, it would be remarkable if styles remained constant. Indeed, they rarely do, except that we are left with the nagging question of why the "classical style" has become a more lasting touchstone for today's audiences.
One might conclude, as many do, that it is simply superior. Such ideas make little impression on me... they involve a particularly contorted view of evolution in music (to dismiss earlier styles) gone awry (to dismiss later styles). What is more, the core Carnatic repertory, for instance, dates to precisely the same time period. Coincidence? I am left to examine other factors in what I will term a "phase lock" between the usually loosely coordinated development of musical style and audience expectations. In short, the impetus for artistic growth found its outlet in the rapidly increasing market, directly balanced by the seductive effect of "high culture" on the rising & impressionable middle class. This suggests a reciprocal relationship between social & artistic change. What is more, the increasing market for noninteractive music naturally leads to a bifurcation between the concerns of the elite (the "original" audience) and the general population. That these concerns managed to align for any significant period must be seen as an anomaly driven on the one hand by the thrill of a larger audience and on the other by a desire to emulate. The result was an intensification of constraint per se, as increasingly restrictive measures were taken in an attempt to recapture the initial magic of this alignment of forces. More recently, popular culture itself is playing out a similarly pathetic scenario, in an attempt to recapture the singularity of style which accompanied the initial appearance of mass electromagnetic media.
Of course there are many good reasons to adopt constraints, whether explicitly or implicitly. Musical expression relies on listener expectations at some level, and so necessarily implies constraint. Many modern compositional techniques involve building those expectations within a single piece, or within a small body of work, requiring both a basic pliability in forming expectations and of course closer attention. More restrictive forms, such as canon or fugue, are attempts to use voluntary additional constraints in order to create a heightened expression. At that level, constraint can be an artistic statement on its own. Similar remarks could apply to constraints of duration or forces, and indeed those kinds of constraints help to forge sub-traditions within particular technical styles. If we put these elements together, especially as combined with stylistic elements, each piece will have a set of interacting constraints. Analyzing the way constraints are articulated or expectations are built can sometimes be a very useful step toward understanding a piece. In some postmodern styles, this process of ascertaining specific constraints is essential, because the locus of activity takes place with the abstract constraints themselves as actors. In any case, one invaluable element for good musician-audience interaction is being on the same page with regard to expectations, or more basically the way constraints are to be articulated, implicitly or a priori. At that point, direct apprehension can occur.
From this discussion we might tentatively conclude that the factor which defines a tradition as "classical" is the consistency of its constraints, especially when a certain level of notional complexity is maintained. Indeed, it is a common set of predetermined constraints which also allows a classical improvisation technique to develop, and of course in that case it must be possible to articulate both constraint and substance in a single pass. This is more broadly indicative of the way constraints function in art, in that the freedom of improvisation dictates a reciprocal constraint on the way expectations must be built within a single performance. Ultimately it is the constraints which define the tradition.