Will be two reasons, firstly, the app developer has tagged their app for a bigger screen than the Pulse Mini, the second is that the Pulse Mini ROM is not fully recognized by google market, so many apps are missing.
In either case, you might be able to get the apps you want directly from their developers website. If not, search for the apps on the web. There are various places where you can find the apk files. Just download to your computer, move them onto your mini SD card on your phone, and run the installations from there. I found that method useful for many free apps that my pulse mini couldn't 'see' on the market, and they run perfectly well once installed.