Nobody knows. This is one of the things that makes efforts to regulate UPFs frustrating.

Some people say that it is lack of fiber. Some people say that it is inflammation caused by preservatives and/or stabilizers. Some people say it is the hyperpaletability, which encourages people to overconsume calories. Some people say it is just the high amounts of sugar and salt. And it is unlikely that all of these things contribute equally such that they should all be regulated the same.