Why are flickering LED driver circuits cheaper than flicker-free circuits?
Basically the input cap across the HV DC bus is hopelessly tiny so the bus voltage pretty much tracks the envelope of the mains. 

This means that there is no need for an expensive HV electrolyic and that the power factor is actually quite good even with a simple minded converter like that. 

If you want flicker free, you need to hold the bus voltage up sufficiently to avoid the thing dropping out or regulation, and that means a rather large electrolytic. If you also want good power factor, that means an additional PFC stage (at least for simple designs).