If they didn't release them on the PS3 at all then they either...
1. Were paid to do it exclusive
2. Didn't think the time/money spent developing a PS3 port would be worthwhile.
3. Didn't have the resources available to them.
4. Didn't think it would make a profit on that system.
5. Didn't want to commit to too many projects at once.
6. Didn't have the time available to the developers to port to multiple consoles.

Or some other business reason. There are always business reasons. Always.