I blame the Reformation ... It's easy to characterize bathing as decadent, luxurious (in the old sense of the term) catering to vanity, conducive to sins of the flesh, and so forth. Given that Protestants covered their heresy with a very slavish veneer of moral righteousness, in part a legitimate reaction against late-medieval and Renaissance excess, it's easy to see how bathing could have gone out the window in many parts of Europe. Also, the Reformation triggered widespread, long, and very devastating wars (always a sure way to disrupt social customs) and economic changes. The rich grew much richer on spoliated Church property, and the poor grew poorer, as kind monastic landlords gave way to greedy Calvinist-minded and socially-striving types, usury became even more prevalent, and enclosure and, still later, the Industrial Revolution forced the peasantry into widespread destitution. When you have to slave away for most of your day to earn enough gruel for the stew-pot, bathing becomes an entirely secondary consideration, provided you can afford to spare the extra wood for the fire and have the energy to make additional trips to the well.
I know my own personal experience tells me that when I'm stressed and busy (as, for example, during finals week in college) the fifteen minutes for a shower and shave suddenly becomes much more difficult to squeeze in. I would be inclined therefore to the simple answer: During the Reformation and subsequent centuries, life simply got harder.
If busy men in the 21st century find themselves changing from a daily shower habit into a thrice-weekly habit, how much more might folks who, before, were only inclined to bathe thrice-weekly, or even once a week?
Makes a fair amount of sense to me.