Do Certain Droid apps only show up on certain phones?

My question is about a Droid program and the Cell phone, and cell phone provider. I have a Verizon Motorola Droid, But a coworker from work has Cricket Cell phone that runs on Droid. I was Trying to help her with her phone by installing cool apps. I notice then that some apps that i got did not show up in her DROID Market Place like the Tango video call App. I thought that any app can show up in droid market Place on any Device that runs on Droid? Why is that? Is it the Cell phone it self? or is it the Cell phone Provider which in this case is Cricket?