Because christians literally forcibly converted everyone they could, and killed anyone who refused. Also it didnt die completely. Most people just worshipped in secret, and in some more rural areas it survived. Aswell as mixing with the christians culture and influencing them. Like Santa Claus is just Odin. A lot of Jesus's associations came from Apollo because Apollo was super popular in the roman empire and had lots of temples and when the romans converted them to churches a lot of the preists were like ok well im a preist of apollo thats what i know how to do so ill just keep doing what i always did but say jesus now instead. Laurel wreath on head, frankincense, etc. All Apollo things.

Most holidays trace their roots back pre-christian too. Christmas, Halloween, Easter, all can be traced back to old pagan traditions.

A big part of like christianity in general is converting others. Not all religions are like that most arent actually not even all christian sects are. Its why it spread so far cuz people who follow it actively try to spread it. Even today religious christian groups travel around the world trying to convert people. In areas they colonized they did the same, convert, or die tactics.