Mostly because VLAN 1 is treated differently by a lot of switches. I've seen some that won't tag VLAN 1 traffic no matter what you set in the GUI, and some other strange & incorrect behaviour. It's easier to just avoid using it altogether. Also because it's the default VLAN, it's pretty easy to inadvertently end up with untagged traffic all over the place that you weren't expecting, or ports that can get on networks they shouldn't, ARP broadcasts crossing VLAN boundaries etc.