I think a lot of people here are just missing a very simple point:
For every dollar that Activision is putting into PS3 games, they are getting a substantially lower return than they are getting for every dollar they put into the Xbox 360 or the Wii. It's that simple. The logic is kind of obvious; If they put those dollars into the other systems, they stand to make more money. This is for various reasons:
1. Developing for the Xbox and Wii is significantly cheaper than the PS3. The PS3 is a huge hassle to develop games for because of the architecture of the system. Developing for the Xbox is not much different than developing for the a PC. In fact, it's easier in many ways. Developing for the Wii is not very different from developing for the Gamecube so there is already a built in level of experience at most companies.
2. Sony charges significantly higher royalties on PS3 games, largely because the developer has to pay Blu Ray royalties on top of the actual PS3 game royalties, which are already higher than Microsoft or Nintendo's.
3. There are (very roughly) 20M PS3s sold, compared to 30M Xbox 360s and over 50M Wiis. That's substantially lower playerbase to sell your games too. before you yell "BUT THEY ARE JUST PORTING THE GAME!", look at points 1 and 2.
4. PS3 has less games sold per unit than Xbox or Wii. PS3 owners simply buy fewer games. Some of this can be chalked up to the fact that a lot of people initially bought the PS3 as a cheaper Blu Ray player, because at the time it was very cheap for a Blu Ray player.
Each one of those points may not seem too terrible, but put them all together and it adds up to a significantly lower return on investment in PS3 games compared to Wii or Xbox.
Too many people seem to be hung up on the fact that Activision IS making money on PS3 games. That doesn't matter. The important part is what the RETURN is compared to the other systems. Activision has a limited amount of money to spend developing new products. If that money is better spent making more games for the Xbox and Wii, or simply beefing up the games that they are already making, resulting in higher sales, then that is CLEARLY the smarter business decision.
To make it as simple as possible, imagine that you have three options to invest your money and all are basically riskless. Each one has a different return rate, but they will all make you money. One is going to return 5%, and the other two are going to return 10%. Would you split your money up three ways or just put more money in the two 10% investments? The answer is obvious.
If Activision can make more money by putting their PS3 money into Xbox 360 and Wii, that's what they will do, period. You can fanboy it up all day, but this isn't about which system has games you like better. This is about what makes Activision money.