Here is the speculation of a person so marginal as perhaps to be crazy:    The popularity of a tune, at any rate since (say) 1940, has had very little to do with the tune itself. Very large numbers of customers are too cowardly to follow their own tastes; they are concerned first of all to agree with what their peers are buying. Such people may not be a majority, but they are the most reliable & profitable customers (as drunks are the most reliable & profitable customers of bars), so it is they to whom the market adapts itself. Popular songs form clusters in quality spaces because a tune that resembles tunes that have already been popular seems like a smaller risk to the manufacturer and to the customer, both of whom would prefer to be participating in a fad -- the first for profit, the second for comfort.