Because the people who use the planes in any measure is fairly small group, so they get the luxery of a fairly fleshed out setting. Most people don't use the planes, so creating dozens and dozens of variants (rather than continuing to flesh out the ones they have) is pointless. How many variants of Hell does one need?
I mean, its the equivalent of saying "I love gothic horror, but I hate Ravenloft. I wish WotC would produce a series of non-connected gothic realms I can mix and match." Or "I love Arabian myth, but hate Al-Qadim." At a certain point, it behooves them to try to do a few things well rather than produce endless variants of the same material.
Speaking of which...
Which is why this argument always cycles back to "Why doesn't WotC produce products that I want!?"
The problem is, this argument can be stretched out ad-infinitum. Someone somewhere is going to dislike something about D&D, so they shouldn't make it "core". Barbarians, Flumphs, Dragonborn, Monks, Tieflings, Death Knights, Halflings, Rogues, Orcs. If it exists in D&D, someone has wished it wasn't in the core books.
WotC gave you the resources, yet you complain they didn't build them already for you to your exact specifications. You have a pre-made set (one thoroughly detailed, others lightly sketched) and you have the tools to build your own. What, that this point, would make you happy other than an Official Hussarscape Sourcebook?