I think it's a rather simple problem. No profits.
When you're trying to market high quality aftermarket performance parts (I assume this was their goal), they have to consider how much of that market share they have. Obviously, there aren't that many consumers that will pay their prices while there are 'similar' items massed produced and sold @ 1/3-1/2 of the price. Little market share, high labor and overhead (assuming they R&D, manufacture their parts in the US for better QC), fierce competition = no profit.
From a business standpoint, APC is one of the luckiest or smartest company in the car industry from the last decade. Entering into the industry at the right time, right place with products that 'attract' new customers. I bet it should be a multimillion dollar company by now.