Some ISPs don't provide static IP addresses (like mine) because they don't support business users, as business users can sue them for downtime, and (apparently) the majority of people using static IP addresses are business users.
Seems like a shit way to approach it IMO but that's probably the reason. If it's not the reason, it's worth giving your ISP a call to ask them if they can provide a static IP, but failing that, get a new/better ISP.