1) One has to find out if the planet is tectonically stable. This requires recon-in-force missions. And lots of Confed offensive work.
2) There's no guarantee the Behemoth is big enough to go through a wormhole (but in all due fairness, some of the bug ships are big)
3) A Behemoth size vessel attracts attention. Not exactly useful for a scout mission when the objective is not to be seen. No one is stupid enough to send a Behemoth sized cannon into the worm hole for mere recon. Imagine the radar signature this time emits.
4) The Behemoth was a gas guzzler and an energy hog, and requires lots of logistics to operate. Kinda would be dumb to send the thing into the middle of nowhere through the wormhole, and find out there's no nearby planets with superduper Hydrocordaline gas to keep it running huh?
5) Behemoth needs a supporting fleet, it cannot operate with minimal fleet strength. They will not commit it until there's enough forces in the sector, and putting enough forces into enemy space is going to piss off a bunch of bugs. Only fools charge the most defended sectors head on. But the Behemoth is so big that's the only option eh?
6) Gaining control of the other side of the wormhole is a goal, not a step in the ladder. Gaining control on the otherside is already going to result in heavy casualties.
7) I'm going to put a big bullseye on your ass and declare it open season, if you make one of your WC contradictory "fan designed" Mega-Super-Giga Behemoth in your lame attempt to expand Wing Commander in the name of creativity and diversity.