Ansiblex jump gates sound like a cool idea. Hey sweet I can link my alliance space together by placing a couple of gates and have everything close together. Super convenient, right?
Being so convenient is exactly the problem. It promotes umbrella spaces in which power of a single fleet is projected huge distances. It creates a mismatch between distances needed to attack and defend, nothing is more than a couple of jumps away for the defender. Large scale invasions can also use them for offensive purposes of course, but smaller operations and roams suffer.
DISTANCE is a good thing for Eve it creates opportunity for achieving local superiority and manoeuvring. Having numerical superiority has always been a part of EVE but that numerical superiority should be at a location, not omnipresent without travel time.
Jump bridges had the same problem which was tried to remedy using jump fatigue (which IMO is a really subpar solution). Ansiblex jump gates only have fuel costs, which means virtually unlimited power projection with enough resources. The only upside is that the gates are relatively easily destroyed.