While I forget the details, I think the actual canon is the the other Hellgods could not destroy her, but COULD bind her to a mortal human, and she'd then die with the human.
After that, I have to go to headcanon. So many prophecies were coming to date around this year rather than business as usual, so I see it as a "high tide of magic." Maybe the stars or right. Or our planet slipped through an especially thick concentration of dark matter, if you prefer (science and magic get mixed a lot in the Buffyverse, quantum mechanics even brought up in Buffy season 1, the ep about Marcie going invisible), but whatever, the Mayor (after over a century) could finally turn into a True Demon while the Master almost escaped and more than one apocalypse cult became exceedingly aggressive and all that...and Glory began to manifest within Ben, but probably not able to hijack his form right away. Because now she could, which was not planned by those who bound her to the form of Ben.
(But also that Ben sensed her and she was a voice in HIS head before she became real...hmm, kind of like how Marcie felt invisible until she really was. Gods, the Hellmouth is more disturbing than I realized! But he'd been studying the occult at least for awhile, which I'd expect someone like him to do once he accepted this was happening.)
I presume he's a new arrival in Season 5, especially as he gets fired for not showing up too much (as he's Glory), and no real mention of his long experience there (I'll ignore his age, I mean look how young Xander was before put in charge of guys much older than him, or the barely more than kids Initiative, who were special forces rather than newly recruited grunts).
Maybe the minions (who feel compelled to have a god to protect them, lots of humans are obsessed with finding a "protector" that actually abuses them and are more miserable without one than with one, and that includes men as well as women wired that way). Or maybe Glory sent out a "distress call" that they picked up on and helped her to resume her form (alternately, she used a brief moment of escape to conjure and bind them to her service, maybe from her old dimension)...and of course get to a Hellmouth where the barriers are weaker (and probably help her manifest more and longer, whereas in Chicago she might only get a couple of minutes, and at greatly reduced power).
I'll spare you my headcanon for why Glory takes the form she does.