My answer is exclusives don't sell badly on Ps3, you're simply comparing the wrong exclusives as you're using genres instead of franchises.
Let me explain.
Playstation's biggest exclusive now is actually GT5 - not an FPS. Assuming that sells around 10M (which is more than possible) then that's the PS3's Halo. You need to compare franchise sales not biggest exclusive FPS.
You're making the mistake of assuming because an FPS like Halo sells 10M on 360 an FPS on PS3 should do the same.
This is wrong. You might as well wonder why Forza 3 on 360 sells a fraction on 360 of PS3.
MGS4 has sold over 4M so far, God of War is steaming towards 2M already, etc. That's PS3's Gears of War, etc.
The reality is both consoles have a small number of big exclusives and so far the 360 has seen more relased vs PS3 (i.e. GT5 is still to come).
However soon enough - probably by the end of 2010 unless GT5 slips - you'll see both have a couple of big exclusives (Halo, GT), some decent big selling exclusives (Gears, God of War, MGS, etc) some decent mid selling exclusives (Mass Effect, Resistance, etc.) and roughly equal sales (in relation to install base) for most multi-platform titles.
So basically I think you're completely wrong to assume PS3 exclusives sell badly because you're comparing the wrong exclusives.
Try to be reasonable... its easier than you think...
