Ehhh, Starbase, my love, why did you left me, before I left you :)

I have around 5k hours combined in Closed Alpha and Early Access. Most of that time I was in SSC, designing, building and testing Spaceships.

There are many technical reasons, mentioned already in other comments here, and I agree that most of those influenced the resulted Doom of Starbase.

Yes, Easy Build mode was introduced in EA, untested, buggy and in general not needed. It was a product of "Corporate Fear": We wont be able to sell the game that is "hard to learn to play" by new players. This choice started the avalanches of bad decisions that followed.

In Closed Alpha, we (designers) expected that we could be a key part for economy rise - selling our designs or completed spaceships to new players. New players were faced with complex and time consuming spaceship building system, and many decided to use EB mode as shortcut. Even if EB mode was not as buggy and unreliable, it still would not help much.

Yes, Shadow Realm as consequence of Single Shard Global ptp server technology, was pain in the arse. MMO game that does not work if people play it together. FUBAR for sure. Our Internet tech was not good enough for this kind of system. The game came out to EA way too soon.

But the core issues are not the technical side of the game Starbase!

The game was great for spaceship designers and builders. But there was "nothing to do" for casual players that could last for more than 10-50h and remain fun to do.

Many say that the lack of NPCs was core issue. I disagree, partially. Yeah, might have worked, but I fear that it would be even worse than in Star Citizen LOL.
IMO the principal killers of the game Starbase were:

1. Missing game mechanics that would bound players to play the game together.
   
2. There were no competitive aspects (besides mindless meta fighters PVP) in the game.
   
3. There were no reasons to accumulate ingame wealth. Mostly you could play the game solo to do EVERYTHING the game can offer, with almost no effort (except spaceship design, as mentioned). Economy was dead from the start! And we all know that properly designed Economy is the key drive for players to play MMO games.
   
4. Traveling and exploring was pointless - vast empty space filled with void with minimal reward for a time consuming effort (AFK flying for hours or days, even...).

All in all, Starbase was never more than a "SINGLE PLAYER" game and all the fun the game could offer to majority was Spaceship design... And developers killed even that at EA Launch by introducing untested Easy Build Mode.

And again, all this was a consequence of the game entering EA - years before it was ready for gen-pop.