Space isn't all that different from water, so Zeon didn't have that much problem developing amphibious suits (also they have ocean colonies). The Feds had a late start on mobile suit development, so they focused most of their effort on ground-use and space-use suits to retake Zeon-occupied territories. This turned out to be a great idea in the end because all of Zeon's amphibious toys ended up being useless when the battlefield moved into space.