The history of American football begins with rugby football. The game evolved as equipment changed, player skill evolved, and most importantly rules changed which produced subsequent changes in equipment, skill, and strategy.
My colleague Dick Wagner often asks the question above, and demonstrates how small changes can accumulate to such an extent that one day you are playing a totally different game. He then, to my mind, brilliantly flips the quesiton to how constitutional democracy became social democracy in the 20th century. Small changes accumulate and we end up playing a different game in the political arena just as we did on the field.
Constitutional democracy had puzzles and paradoxes, but social democracy has it own sets of serious puzzles and paradoxes that must be addressed. Simplistically, I think you can think of the puzzles and paraxdoxes of constitutional democracy as relating to incentive alignment issues, whereas the puzzles and paradoxes of social democracy are epistemic in nature. In both instanaces, critical analysis requires that we take an institutional perspective --- institutional problems demand institutional solutions.
I tried to stress this epistemic point in a recent review of Jack Knight and JIm Johnson's The Priority of Democracy. From some of the comments, it appears I wasn't as effective as I might have hoped at communicating my point about the epistemic constraints (as well as incentive issues) that must be addressed if we are going to have robust political institutions.
BTW, Alex Tabarrok has an excellent discussion of the issues of the cognitive capacity of democratic institutions at Marginal Revolution.