I think it has a lot to do with the fact that I don’t know anyone who married young. I have one friend who married in her late teens/early twenties (she may have had a shower but I didn’t know her then. I’ll have to remember to ask her.) Everyone else in my circles has been at least in their late twenties, if not early- to mid-thirties, by the time they married (or they haven’t married at all.) When you’re reasonably financially secure and you and your partner have owned a home together for several years already, showers just seem unnecessary and gift-grabby.
Heck, I don’t even see a lot of big white weddings, which I think is also the result of people waiting until they’re older to get married. Most of the couples I know, DH and myself included, have chosen to have smaller, more low-key affairs. Courthouse weddings, destination weddings, in-home weddings... I can only recall three that followed the traditional style people think of when they think of weddings. And I don’t know anyone who has married in a church.
One of those traditional style weddings was for a good friend of mine. She was 36 at the time and told me she didn’t want that type of wedding, but they were trying to appease the older generations in the family who wanted the big wedding (80 guests). She would put on the dress and get married in front of an audience, reception to follow, so long as she could cut out the shower, bach parties, speeches, dances, bouquet/garter tosses, and whatever else she said she felt silly doing at her age.