Matt hit it on the head. I have found that many model truck builders often fall into a few categories. One group builds for fun and does not fuss too much over details. These builders will typically build box stock or find some donated parts here and there to switch things up. This type of builder is not looking to spend $60 on a set of rims and tires because kit wheels work just fine for them.
Other builders fall into the "it has to be exactly what I am looking for" category. These builders will not buy a part unless it has every exact detail that they want. I had a builder tell me that a tire I cast is perfect, except that it didn't have the brand name that he wanted on the sidewall so he could not use them. Getting one of these builders to buy an aftermarket part is very few and far between because you have to get lucky enough to have made the exact part with every detail that they want for a specific build. Typically they will not use the same part on multiple builds, so sales are even limited there.
Most aftermarket sales seem to go to the in-between crowd, which is a niche of a niche. Now return on investment is tough. Take a tire for example, I may spend 10 hours drawing a tire in CAD, then spend $45 to have a master 3D printed. Then I have to spend another 3 hours making a mold (which uses $30 worth of silicone). Now I have spent $75 and 13 hours before the first tire is made. At $10 per hour, that would be $205 that would need to be recouped. When all is said and done I have to sell around 150 tires to break even at $3 per tire. I have tires that have been very poor sellers, I even had one tire that I was never able to sell a single one. It seems that it is a gamble as to whether it will be popular or not.