The only company that really buys 4:3 panels on a regular basis is Apple and they probably buy up most of the capacity for them. On the other hand, if you make 16:9 or 16:10 panels of certain sizes, you'll have an easier time selling them to any number of manufacturers who make Android tablets. For the people buying the panels it also makes it easier to shop around.
Basically if you want 4:3 panels, you either have to wait around for leftovers, compete with Apple for them, or have a custom order that will likely cost more than other panels.
The Android market is already pretty cut-throat so added cost is almost always avoided. I would imagine that most companies are leery of the potential risk of such a device not selling well compared to other tablets in which case they're out a lot of money.
I think Archos recently came out with a low cost 4:3 Android tablet using some of the old iPad display panels before Apple switched to the higher resolution displays. HP has a 8" tablet (Slate Pro) that's 4:3. Beyond those two, I haven't heard of any recent 4:3 Android tablets, but there are usually a few around at any given time.