I thought the LAA had total engine power, all up weight, and seating limits. It’s only recently that 4 seaters were considered. The Cri Cri is, to say the least, unusual. some would say a novelty type.
It has been done before (Philips Speed Twin)
Yes, but they gave up on the “homebuilt” part of their project long ago and decided to go the production aircraft way. There will be no plans or kits for this aeroplane.
Regarding the original question: My guess is that it is simply about money. The engine (+ propellers, tanks, fuel system, engine instruments) is by far the most expensive part of every/most homebuilts. A twin effectively doubles this cost factor.
It does not help either, that the market is flooded with pre-owned light twins that can be bought for less than what a single of it’s engines is worth.