I found this question last night and I've been pondering over how best to answer it. It's tricky, because to an extent I think you're begging the question - while men's dress has kept to the same basic parts, there's still quite a lot of differences in men's clothing from different eras. You would recognize men in business attire from 1875, 1905, 1945, and 1975 as being from very different times if they were lined up on the street, probably.

(You might want to look at The Cutof His Coat: Men, Dress, and Consumer Culture in Britain, 1860-1914, by Brent Shannon, particularly the linked page, which addresses why we perceive men's fashion as having been unchanging.)

But there is a lot less extreme, cyclical variation than in women's clothing, even if you began looking at menswear from the reign of Charles II, when breeches/coat/vest were codified as court dress. What is basically comes down to is the sexist belief that women were inherently frivolous, fashion was frivolous, and therefore fashion was fit only for women.

At the same time, middle and upper class women were expected to consume fashion in order to show their family's income and their own taste. Because fashion was an industry subject to competition, the designers had very good reason to constantly change and promote new trends.