content: Everything this guy said, plus IPv4 depletion. In ARIN (North America) there are no* IPv4 addresses  to assign to a new ISP, and RIRs frown on using out-of-region allocations without proper usage in the original region first.  So, I couldn't go to AFRINIC (pretty much the only RIR with a decent amount of IPv4 left) to get IPs to use in the US or EU, if I was just starting/it was my first allocation.  One could try the second hand market - older companies that got IPv4 space before RIRs and actually own the space may sell, but that shit's expensive as all hell.  As sad as it makes me, an IPv6 only ISP wouldn't do well; too many sites aren't dual-stack yet. *It's more complicated than I want to type here-but for the purpose of our discussion, they're out.