Probably because the NR AR build will be significantly lighter, since he wants a non restricted rifle he can also do 3-gun with the weight savings probably makes it worth it to him. That doesn't make one better than the other, it makes one more suitable for a specific persons needs.
Anyone who builds a MV-s with quality parts and a good barrel that is capable of decent accuracy is not going to be spending $800 like a regular old AR like you compare the ACR to. The costs will be around $2000 even for a semi econo build.
You don't like the ACR? Well you're one of the few, most people simply think it's too expensive which is code for I really want one but I'm too poor to afford one, or my wife won't let me.
I've owned almost every non restricted 223 we can own and the ACR is still in my collection after my Swiss Arms, HK's, and Tavor have all be sold off. I don't think there is a better rifle available to us but due to the weight of it with a NR length barrel it isn't the most ideal rifle for certain shooting disciplines. That doesn't make it a bad rifle, it just makes it not the best at everything, there is always a trade-off, and that's why some of us choose to own multiple 223 semi auto rifles.
You mentioned price and weight as the things you didn't like, well compare the price and weight to most other NR rifles on the market, hmmmm... very similar.