GLSDB was already approved in February 2023 before even ATACMs were approved. So this is not from the "dribbling" that was referred to above thread (a lot of that is actually deliberate frog boiling discussed up thread to push Russian red lines without risk of escalation). As per the wiki article, the delays stemmed from problems in development and production. The original estimate was delivery in up to 9 months (so around November last year) with hope it can release earlier, but it went past that due to problems within the manufacturer.
There were only two bunker busters used during the whole war.
GBU-28 - Wikipedia
Instead there are 12 of these in each M270 (which Ukraine operates around 15) and 6 of these in each HIMARS (which Ukraine operates around 30) and they are also making them for Taiwan (order of which had to be delayed to give priority to Ukraine), so it's going to be a long term supplied weapon, not something cobbled together for almost one time use. Ukraine seems to be expecting around 200-300 units delivered for each batch and in the long run thousands would be made.
GLSDB: Ukraine Will Have to Wait for These Long-Range Missiles – Here’s What They Can Do
I think the development that needed to go into this is going to be a lot more, and making a production line that can pump this out consistently is going to be way different. Given there is potential long term business, the earlier the manufacturer can make this without delay, the better the business, so there is no defense industry incentive for delaying this. As a sanity check, I also considered if it was an issue with cannibalization, but the more expensive longer range ATACMs is made by Lockheed Martin, the more expensive shorter range M31 is made by General Dynamics, while this is made by Boeing/Saab.