I didn’t know this controversy existed, but put me down as a migrationist.
People have been moving around in the migrationist way for as long as we have records. Why in the world should we think people before we had records behaved differently?
Actually, I can answer my own question.
Most archaeologists and anthropologists are culturally and politically liberal.
They want desperately to believe there was a peaceful Golden Age during which people didn't spread by war and violence, with more efficient cultures destroying those which weren't as good at killing.
So they are constantly reading these memes into the limited evidence, which is usually overturned pretty quickly with evidence of massacres, human sacrifice, cannibalism, etc.