It's to allow the organisers plenty of time to sell tickets, particularly in the case of the games between tier 3 & tier 4 teams. Plus they can firm up plans for where teams can be based in terms of training, logistics regarding ticketing, grounds being used, etc). While fottball can be guaranteed ticket sales whoever is playing at a World Cup, it can be a harder sell for tickets to, say, USA vs Tonga, so having the pools generally decided well ahead of time, the organisers will know most of the fixtures & can adjust game locations to best suit the expected attendances.

I was wrong earlier in saying the summer rankings would be used. Apparently it's the rankings at the end of the November International window.