That's like saying, "This deli has too many kinds of sandwiches."
If you can find the sandwich/dragon that you need quickly enough, the mere existence of others on the menu should not have much of an impact, should it?
Now I need to stat up a sandwich dragon.
Ah, but no chef can make 100 perfect sandwiches right? What if this deli does have a huge menu, but only 5 of the sandwiches are actually good while the rest are just kind of slapped together?
I do agree that more dragons with excellent, different stats would be great. But considering the size of 5E books, there can only be so much content in it without pushing the books beyond their "acceptable size" for WotC publishing. So more dragons competes with the remaining dragons getting more lore or lairs or PC options, etc.
Adding two more dragons in this huge book, doesn't bother me. But WotC has to draw the line somewhere, otherwise we would be seeing pink, brown, yellow, catastrophic, adamantium, orium, mithral, obsidian, electrum, pearl, gray, orange... the list goes on and on and on. So some of these got to be removed, and the more you add, the more they compete for other content. So is a book of just statblocks of dragons better than a book mixed with statblocks and other material?
So you know, not as simple as "more choice is always better."