Because it involves some expensive changes in the manufacturing process? Someone already explained this in another thread, or maybe it was on the indiegogo q&a-page, i forgot about that…
And why they didn’t choose DIMM-slots, i can only guess: maybe it’s about the size?
Edit: Another answer that was posted on the indiegogo-page: “This means redesigning the board with new pads for another RAM module. I guess we (2GB backers) will pay partly for the board redesign and partly for the RAM chip.”