And I should also say that the same thing which explains the Gbaba's [relentless alien-killing] behavior explains the stasis in their technological development which was observed by the Terran Federation during its losing war against them.
So - while RFC's not telling, we can surely speculate - what would explain both of those and is consistent with all else known about the Gbaba?
Here are four notions, somewhat related:
One - The Gbaba hate and fear the unknown. Unknown aliens must die; unknown space must be avoided unless going there is necessary to kill aliens; unknown technologies must be avoided unless they are necessary to kill aliens (or get to space with them). It could be that the Gbaba only started exploring space and developing hyperdrive once their SETI program (to watch out for threats) uncovered them. Maybe this general xenophobia is learned (on the basis of horrible experience with aliens, theoretical studies, Gbaba religion), or maybe it's somehow genetic. For that matter, there's no reason to suppose it's got to be either/or.
Two - It could be a more specific fear that underlies both. Maybe there's a technological plateau for Gbaba intellects, and getting beyond it would require the use of artificial intelligence. But if artificial intelligence is anathema to them, they may also regard any other aliens as potential creators, users, allies, or tools of AI's and have to destroy them. Why they do not explore more actively is still a question here though - there's no way AI could be required for that, and it's a reach to suppose that distant scouting groups would get infected with it somehow.
Three - spoiler alert for Empire from the Ashes future readers
- the Gbaba are more-or-less Achuultani: they're enslaved to artificial intelligences that smack other lifeforms as threats to them, threats to their dominion, and/or a necessary pretext to retain their control. Technological stasis is maintained to preserve that precious and adequate (to the AI's) status quo; exploration is avoided to keep the AI's all on the same page; and keeping them all on the same page may be part of the technological stasis too - advancing the tech base may require or allow a next generation of AI to which the existing ones would be enslavement-fodder.
Four - a variation on one - the Gbaba hate and fear what they don't control. Aliens are not too subject to control - they have to die. Distant space is tricky to control - go smack it and its denizens when and only when they (perhaps including their EM emissions) intrude on your space. Further technological developments will upset the Gbaba status quo, whatever its basis is, which may not preserve the control their ruling class has or perhaps even Gbaba control of themselves, in the case of "monster AI". They have in effect adopted their own Proscriptions (albeit at a vastly higher level of technology), they have kept to them for thousands of years, they don't write dispensations or take bribes, and they impose pre-emptive capital punishment on anyone else for the possibility of violating them. (Or that and all the other ways they may escape control.) This one plays into RFC's comparison of Clyntahn to the Gbaba.