Two-letter codes are assigned to anybody on request, but three-digit codes are assigned only to full IATA members.

The three-digit code is used primarily for ticketing (it's the first three digits of a ticket number), and as an airline you only really need it if you're going to do complex interop things like ticketing another airline's flights. Most low cost carriers like Ryanair are not IATA members, and even Southwest only joined last year.