I wondered the same thing ... I started buying pipes in the 80's and you could easily find bowl depths of 35mm -- 55.. today everything seems to end at 30-32mm for rhodesian and 37mm for billiards --- at least on the online sites. I am not saying larger pipes are not entirely available but you have to carefully look for them. Also all the pipes seems to be black and sandblasted now ... in the lower price ranges ...The question is why make a smaller pipe?
Maybe people have less time to smoke because they are so busy and that's what sells now.
Or the other possibility is that all the others have already sold and the smaller pipes are what's left over...
Or they are trying to attract another modern group of smokers with the mini pipes ...