Mostly because it isn't as cheap as it used to be. You can buy a 25 fp for about the price of a Cox Baby Bee. It runs on 5% all synthetic or a mix of castor where you need castor to keep that silly ball socket lubed on your Cox engine AND the cox engine would prefer 25% nitro which isn't cheap even if it does sip it. Then, here in the Kansas/Missouri area you have a little 1/2A that is more affected by the wind then a 25 size engine. Most hobby shops carry a good line up of props that will fit the 15-25 size planes where they are iffy on having 1/2A props if at all.
Still, if you have a couple old fuel tanked Cox engines OR the ok cub,norvel, brodak etc engine you can mount it on one of these Kits. They are a blast from the past. Frankly though, things have evolved quite a bit since these kits were introduced and powerplants got scarce and expensive. It just kills the market for them. Really, you're paying 25-35 bucks for a half a kit? In a similar economy not to long ago the generic kits would cost 5-10 bucks and the engines brand new were under 25. The cost of the 25 fp/la hasn't increased that much proportionally nor the cost of the kits. You gotta WANT one of these little guys to make it worth while.
They sure are cute though. Glad I bought the ones I had a long time ago.
Oh, the 25 fp/la engine isn't very finicky either. Throw some fuel at it, flip the prop OR put a starter on it and go fly.