As a Buddhist, I will say that at its core, Buddhism is more of a philosophy than a religion, which is the reason why it has been adopted as numerous syncretic forms across East and Southeast Asia. That being said, though, there are several main reasons why Buddhism tends to be incompatible with Christianity and Western philosophy in general.
The biggest one is the difference between nirvana and salvation, as the former emphasizes eventually reaching a state of "nothingness," while the latter involves conducting morally beneficial physical or mental deeds in order to eventually ascend to heaven. Granted, both are oversimplifications, but it should highlight the fact that one of them essentially involves gradually cutting off ties with society, and the other concerns forming beneficial links with society, although the rules are significantly modified for laypeople.
The other significant issue is how each belief views life, as Christianity tends to have a linear outlook from life to death, while Buddhism has a circular view, as reincarnation theoretically occurs over and over again until reaching another plane of existence. This is also why karma is often viewed in the Western world as a short-term experience, while within Buddhism, it tends to be considered as an innate existence which spans multiple lifetimes. The latter also discusses multiple planes of existence, each spanning more than trillions of years and various cycles, although this is too complicated to discuss in detail here. In addition, the Bible and the sutras have been developed and revised various times by various sects within different regions and countries for millennia, meaning that their contents must be rearranged significantly in order to be compatible with each other.