First off there are more 360s sold than PS3s so that explains why most older multiplatform games have sold more on 360.
Now to why the PS3 exclusives for the most part don't sell like 360 exclusives. Microsoft likes to bundle their games like Halo and Forza. Also PS3 has a larger variety of exclusives, where as 360 is mainly Gears, Halo and Forza. Sony has more exclusives even though they sell in the 2-4 million range except Gran Turismo 5 and Metal Gear Solid 4 for obvious reasons. some people like inFamous, some people like Ratchet some like Resistance, some like Uncharted, KillZone, Motorstorm, Littlebigplanet, etc. There is a lot more to choose from so the money is spent in many different places as opposed to just Halo/Gears/Forza.
That's just my opinion though.
