The simple answer is "if it isn't broken, don't fix it". A lot of the Pros are indeed using 2.4GHz since they are being paid to use them. However, there are some that are more traditionalist and will use the sponsor's radio but with their old tried and true modulation. For example; Lance Armstrong, who has won 7 Tour de France races in a row, rides on the newest and most advanced cutting edge equipment in the world but he prefers 20 year old pedals. Why? It's a comfort zone thing, he feels that it works for him so the team scours the earth looking for old but never used Dura-Ace pedals. Call it Superstitious.
I bet Drake and the others still using 27 and 75 are doing it because it's in their comfort zone. I do know that many other Pros use 2.4GHz and win so it's not a time lag issue.