My guess ... they need the guaranteed cash from popular weekends like W&D, Marathon and Princess earlier in the fiscal year to help pay off actionables like guaranteeing park rental for the dates they have planned, overseas orders for announced things (medals, annual RunDisney merch like the black and red stuff this year).
I think that RunDisney is basically at a break-even point cashflow-wise right now, if they are even break-even to positive. I'm basing this off the merchandise situation - how that in the second-half of the fiscal year, we saw little-to-no merchandise beforehand ... and significant amounts of the W&D merchandise were incorrect and had to reprinted. This just makes me think that RD is having trouble paying bills for these orders towards the end of the year - and with most of the printing and manufacturing done in China, that adds extra shipping time. After talking to people when I volunteered at W&D, the merchandise DID NOT ARRIVE in Lake Buena Vista until the Thursday before Race Week. That's a real quick turnaround to count/inventory merchandise. And it was obviously something that was rushed because of the misspelling.
Anyway. If I had to guess (with my somewhat experience in things like this), that would be why RunDisney is asking for cash upfront a lot quicker this race season.