Why don’t I see the same apps in the Marketplace as I do from one phone to another?

The Windows Phone Marketplace is home to manufacturer and mobile operator specific devices. Some apps found in the Nokia section of the Apps store can only be accessed by Nokia devices. This is the same for HTC and other manufacturers.
The same can be said for your Mobile Operator such as AT&T, T-Mobile, and Verizon to name a few. Applications purchased or accessed through their Marketplace portal can only be downloaded or accessed if you have a device from that specific carrier.
If you switch carriers or devices, then you will not have access to those applications that were specific to their prospective carrier or device manufacturer.