That's because the poorer quality kit or a model by a lesser known company is not as popular or else it would have been done by a mainstream manufacturer and then many AM manufacturers would produce accessories for it.
It's simple business.
For argument's sake, assume it costs Eduard the same amount of money to make a PE set for the new Dragon M1000* as it does to make a set for the Eastern Express BRDM-50*.
Dragon sells tens of thousands of M1000 kits all over the world. It is carried in local and online shops everywhere. Shops can barely keep it on the shelf. The kit is hugely popular and of the highest quality. Builds into a winner straight OOB, but most folks who will buy this 600+ part uber kit will feel they "need" to buy the PE set, tracks, etc.
Let's say the EE BRDM is found only in a few well stocked hobby shops and limited places online. It is dimensionally accurate with some rough or soft details. Only about 10 people have ever seen the kit at their store. It could really use a PE set, resin wheels and some decals. It's had some so-so reviews online, mainly criticizing the limited run quality and poorly shaped wheels.
It costs Eduard the same amount to make either PE set. The M1000 kit will sell thousands of sets for years to come, the BRDM kit will sell maybe one thousand total in its short life. Many folks who own the BRDM kit may buy the PE set, but most toss the kit onto the to-do pile as being too hard to mess with.
Bottom line is that there is more money to be made in making a set for the more popular kit by the mainstream manufacturer.
*not actual model kits, just hypothetical examples.