The app developer chooses what devices his app works on. They don't want a Xoom user trying an app that isn't supported. If / when it crashes, the user writes a nasty review.
I got apps to my nook color by using a couple different apps. One was Hocker, I can't remember what the other one. Essentially it was an app that allowed you to backup your apps to folder on the SD card. It made a .apk for the app. Hocker then lets you send the app via WiFi over to the other device (both devices had Hocker running and on the same WiFi network). You could also transfer via dropbox or using a computer to move the .apk over and forgo Hocker.
The other app was a file manager I think, maybe even astro? I'm not sure though, it's been a few months.
This method does require two android devices though. If you don't have another device to initially download the app, then you are going to have to pirate the app and you won't find help from me regarding that.
HTC EVO 4G LTE - Stock
Samsung Galaxy Nexus - Stock
Motorola Xoom - Verizon - Rooted
HTC EVO 4G - Rooted
HTC Hero - Rooted
HTC T-Mobile G1 - Rooted