Speculating (!), with so many new technologies 'threatening' the market (FB-DIMMs [BGA?], MRAM, CNT, F-RAM, etc), which might also bring new packaging approaches, manufacturers may just be holding on to the price/performance ratio TSOP ('Thin Small Outline Package', for those who are wondering...) allows, considering how highly lucrative (& speculative!) RAM business is. Why TSOP (yet)?
Still speculating, i don't see how BGA can be a cheaper solution to TSOP (just as i don't see LGA being cheaper than PGA...), for two main reasons: first, [RAM] memory density has grown no-where near GDDRx (which, unlike wusy 'said', is made of very stable, fast BGA chips; that's why they're used in graphics; older, slower graphics memory chips, are also TSOP); second - as absurd it might seem (seems to me!) - it may have to do with chip... geometry! See, when scaled down, rectangular chips maintain a better ratio between linear (TSOP 'legs') and areal (BGA 'balls') than square chips; hence, for the same constant ratio, there can be put more 'legs' around a rectangle than around a square. If chip density & pin-count, EMI & bus specs, for instance, are no-issues, than why not to stick with TSOP (even if it brings less reliability than BGA)?
Maybe - aside production costs - these "reasons" are also behind, in more than one way, of the WLCSP (of which i didn't have a clue!) abandon...
Again, just speculating!
P.S.: Although wusy was referring to DDR BGAs while i was referring to GDDR BGAs, i still think the same "reasons" apply.