Up until 1986 they were about $200 more than a semi-auto. Why didn't everyone have one in 1985?
Pick up an old issue of Gun Digest and you'll figure it out quickly. Most guns that were offered in a select-fire version in 1985 were already expensive, and you have to take inflation into account.
If you ignore the effects of the current buying frenzy, the inflation-adjusted price of your average run-of-the-mill AR has gone down by about 30-40% between 1985 and today, and a $200 premium plus a $200 tax stamp also cost more in terms of real buying power. In today's dollars, imagine a $1,200
AR plus a $400
premium and a $400
tax stamp and you're in the general ballpark, and that's for the cheapest
one. Relatively few shooters were willing to pay the price of admission, particularly given the government strings that were attached. Also remember that the Cold War was in full swing, so there were no cheapie "parts kit" AKs on the market to push prices down.
IIRC there were a few abortive attempts in the early 80s to market Sten-like inexpensive compact FA rifles for the home-defense market, but they fizzled.
I find that full auto is not all that usefull. There are only a few times that it is needed by anyone... I think that semi-auto fire is more deadly in about 99% of situations... So as far as full auto goes; I realy don't care.
I believe that this attitude was equally prevalent in 1985 and it goes a long way towards explaining why they weren't very popular.
To use an automotive analogy, it's like asking why more people didn't buy more of the really powerful top-end late 60s and early 70s muscle car variants- the ones with sales figures in the double digits, and collector values well into six digits today. The reasons were quite simple in concrete, practical terms: the more basic versions were already expensive, their power was already ample, and most buyers didn't see the need to shell out an additional 50% premium for one that was even more powerful but also came with much higher operating and maintenance costs.