It's really hard to say without knowing specifically what previous ROM you were using, but there are two possibilities I can think of:
- The apps in question were updated to require some new feature that your device doesn't have.
- (Probably more likely) Your previous ROM had different information in the
build.propfile than your current one does.
The /system/build.prop file contains a lot of information about the hardware of your device (among other things), and it's what the Play Store will use to determine if your device supports the hardware requirements of a given app. It is fairly common for ROMs to use build.prop entries to masquerade as other devices or pretend to support nonexistent hardware for the sake of app compatibility. I would suggest trying to find a copy of your old ROM in zip format then extracting the build.prop from your old ROM and comparing it to your current one. If they are different then you can consider editing your current build.prop to include any missing entries from the old one (there are apps to help you do this). This is also covered in some detail in Is there a way to trick the Android Market into thinking I'm using a different device do I can download protected apps?