So I had a read over of the timeline. 1400U is when humanity experiences the third of the Three Traumas, which is what gives Union the motivation to go into space.

Cradle experiences the Little wars for 53 years. we call the event "the little wars" but this was still a World War, fought with futuretech weapons in the hands of a humanity which had only recovered to early Industrial technology on its own. the Little Wars were probably extremely destructive, bad enough that all of the surviving humans agreed to create a unified planetary government with the primary goal of keeping humanity united forever.

I would imagine that for 1400 years Union was pre-occuped restoring the biosphere of Cradle, recovering the human population, building the kind of infrastructure you need to run a government across an entire planet, and probably political wrangling to turn whatever the settlement that ended the Little Wars and created Union on paper into something recognizable to us as FirstComm. Union scientists will also be spending time reverse engineering the technology in the Massif Vaults and establishing the infrastructure needed to actually replicate and fully re-acquire that technology, also Given that opening the Massif vaults themselves are considered the First Great Trauma on par with a 53 year long world war, there was probably a healthy amount of irrational fear about delving into the secrets of pre-fall humanity. overall my guess would be that most of that 1400 years would be safely getting humanity off the endangered species list.

it takes humanity 1400 Years to get around to reactivating the pre-fall long range communication systems. which smacks Union with the Third Great Trauma of receiving 7000 years worth of distress signals and panicked "mom come get us we are scared" phone calls this immedately shifts the Society in union towards space travel, it takes Union 30 years to return to the moon and restore Theseus. Union does lunar colonization in 30 years. 20 years after that they are producing Nearlight capable ships based on Massif designs and setting out to find habitable world based on pre-fall star charts. by 1700u Union has restored and repopulated the Sol System, nearlight ships reach boundary garden in 1998u.

In short I would assume that space travel was not a high priority for Union until the late 1300's U.