That is another factor, but not one really common in the consumer world (SSD side). SLC prices kept most devices locked to the enterprise market.

Not really sure what the breakdown of SLC vs MLC is for thumb drives. My guess would be that SLC didn't make much of a difference on USB2 devices.

Intel now is moving more towards MLC in general, even for their enterprise products. More focus is on making MLC faster and more reliable these days, due to the lower cost and higher capacities. Building a support structure with more DRAM and better controllers for MLC is still cheaper then building SLC devices.