A reboot allows you to get back to the basic premise with the core characters and none of the baggage from the last 15 years of X-Men movies. The Fox X-Universe has always been a compromise because in the late 90s no one thought you could sell comic book heroes to a mass audience without putting them in black leather and toning the whole thing down.
It also allows a mass recasting of Stewart, McKellen, Jackman, Berry, etc.
Any reboot of the X-men would be pretty grim in multi-coloured spandex. First Class and Avengers has got away with snazzier costumes but only just.
Mass re-casting does not require a reboot. I'm pretty confident that modern audiences can cope. I loathe reboots generally, although I give NuBSG a pass.