Because I believe, broadly, the top tier seems to work best on a week in week out basis when it has 12 clubs at this time. I think you can just about stretch a properly competitive second tier to eight teams if you include the two who have dropped out of SL.
And then I didn't really think too hard about the remainder because the options get fiddly and it's only really improved by either finding some way to raise some park pitch sides from outside the M62 corridor or persuading some strong amateur sides in that area so you get to a more workable number for division splits.
But, for the good of the game, I think you start at the top, where the sport is most obviously 'product' for a wider market and go from there.
I did also say I'm open to anyone else's ideas but the most important bit, really, is consistency and, as others have said, having people at the RFL really focusing and delivering on the lower sections of the game.