Mainly money. How many companies manufacture stone points? What is the sales tax revenue? How many people are lobbying for stone points to be allowed? What is the economic/political incentive behind stone points?
From a common sense point of view, there is no good reason why they shouldn't be allowed, and plenty of reasons they should be. Sure, they CAN fail and cause a lost/wounded animal. So can modern broadheads of all varieties. So can a .270, .308, or any other projectile. But there isn't much economic or political incentive to put any legislative effort into stone points or other "primitive" hunting methods. There might be several thousand interested parties, nationwide, compared with MILLIONS of compound/mechanical pointed/rifle/other modern gun hunters in the country.
There are a few states that allow them, either explicitly or through exclusion (they aren't DISALLOWED), but not a lot yet. There are probably several other states that could be persuaded with some organized effort, and several that never will be.