It's kind of strange to think about why Christians would blend Pagan things in their art. One reason could be that it was a way of slowly transitioning the public's eye to things in their religion. I suppose back in those days it was easier to include things that people were familiar with in the past. It could also be that they liked the symbolization the pagan religion brought to the art world. The only thing about declaring the empire a "Christian" religion is that most of the people were previously pagan is that it's not exactly easy to just up and say okay yesterday we were pagans and now we are going to be christians. That could be why the christians still included pagan elements in their art. I mean no one really knows for sure but that is just one possible explanation. A disadvantage of including paganism in christian art is that it kind of sends a mixed message. It' like hey we don't associate with pagans or anything they stand for but we're going to put things in our artwork that were included in their religion and it's cool. It kind of sends a mixed message. Either way, it's not exactly the best way to try and switch over to a new religion. Things like that don't exactly happen over night, but I guess in a situation like that what the rulers say goes.