Most handset manufacturers don't have a clue about UI design (or have lost it - Nokia did a really good and intuitive UI on the very first GSM phones way back when). Having them inflict their ill thought out cruft on users gives the platform a bad name.
Also, a key selling point of Android is its integration with the Google ecosystem. Having all my contacts and appointments synced into Google by default is the killer app for me. I don't want to have integration with some ill-thought-out Samsung platform that then all gets lost when I buy an HTC phone.