1) Historically, some bands have been concerned about their image and felt that professional-looking photos that painted them in a bad light, whatever that meant in reality, would be more damaging than amateur photos. I don't hear this as much today, but 15 years ago it was frequently given.
2) Concerts with a lot of standing room near the stage already get quite crowded. Someone showing up with a bulky dslr (or even prosumer grade mirrorless) body and a 200mm lens is going to take up quite a bit of room. Prior to the advent of half the damn crowd keeping their phones in the air recording the show for the entirety of it I would also say it obscures vision and annoys people, but now it's really not any worse than that
3) They don't want someone to try and hold them liable if something goes wrong and some expensive camera body or glass gets broken.
On the times I've been able to bring my full camera gear in without a press pass, I stick to as small of a lens as I can and avoid being near the front of the crowd. Thankfully, even quite a ways back from the front of the crowd, a 50mm prime lens will still take some fantastic photos on a real camera vs. what you get with a smartphone. I understand why the rules are in place, though, and I don't really have a problem with them in general.
