I second Eric's comment. At the time Los Alamos chess was invented,
computing power was at a premium. :) A 16 2/3% reduction in piece types
to factor in, a 25% reduction in pieces on the board, a 40+% reduction in
number of places to move...taken together, that's a huge savings.
Combine this with the likelihood that the early algorithms were nothing
more than brute force calculations...