The main reasons are that prices are too high, and supply is low compared to 10 years ago. I think the price increases have kept selling and trading games less frequent amongst collectors. Dealer prices are through the roof, and have been for quite some time. The other aspect is that the quality of work on dealer pins has declined as profit margins decrease due to supplier prices going up significantly. There's no more "bang for buck" from dealers / sellers. Having said that, there are some good deals out there, but they are a lot less frequent.
I think it will get worse. The last two years has been a problem as many collectors have been bitten by WOZ, Predator, TBAG etc etc. Collectors are now holding on to their money and going back to wanting TAF, TZ etc etc - but they are always in for a shock when they see the prices, and quality of workmanship. At the moment, the only affordable games are games like Shadow, Dr Who, Demo Man - these were "C" grade titles years ago. There is also a heavy leaning towards Sega and System 11s as these fit within realistic budgets these days.
There is still a heavy demand for High End Restorations - but these are nearly always "Holy Grails".
Eventually supply will run out, and prices will peak - I still don't think we have peaked - and then it will drop. The industry has always been in one big cycle, so prices will drop and machines will find their way out of collections as we get older and the next generation have no interest.
