Balance, strategic immobility (there's a lot of support equipment and specialized infrastructure needed to build and maintain mecha), cost, weight, battery life, and sometimes materials that can't withstand the rigor of certain conditions (sandy deserts) or endurance that battlefield-ready equipment needs to be reliable in. At least for the projects I've read, like the land warrior system. Hackers aren't really a problem in the real world (military equipment is generally fairly isolated so it's difficult to get in in the first place).
If you can plug them in with a long cord, though, many work fine, so something like the loader exosuit from Aliens would actually be a ready-in-the-present.
