Traditionally the reason why there is always a disproportionate split in development has been that one console's expected return on investment always looks dramatically better than the others.
Total Cost = Expected Development Cost + Marketing Costs
Return = Expected Sales * Per Unit Profit
Return on Investment = (Return - Total Cost) / Total Cost
Fairly soon this calculation (and the variations of it that are used by most publishers) will start looking very ugly for both the PS3 and the XBox 360. The combination of higher development cost combined with lower expected sales (due to smaller userbase) has a massive impact on this equation.
Unless Microsoft can reverse their sales trend soon, lower development costs, or raise per unit profit on every game it will become very difficult for them to attract new exclusive third party development.
