I talked a bit in the January 19th Weekly Dev Log about issues we encountered with our upgrade to Unreal Engine 4.18, which you can read below. We identified and fixed the issue where the game wouldn't boot, but ran into a deeper issue which results in the game never loading past the main menu. Our test ran for 8 hours and it never loaded a level in that entire time, the entire game sitting at 0% CPU Usage and non-existent disk usage.
This issue goes deeper into the engine, as it's something Epic broke. The Linux Beta was a side project by me simply to see if we could built Tower Unite on Linux without doing anything extra right now. At the time it worked good enough for us to decide to release it in the beta format it was. Unfortunately, it's not our main focus at the moment, and I personally don't have the skills to resolve the issue, I've exhausted any attempts for myself to look into the issue.
If/when we upgrade to a new Unreal Engine version, such as 4.19 or 4.20, I'll try building it again to see if they fixed the issue, from there I would then update the steam depot and hopefully it would all be good. Until then however, the Linux Client is discontinued. As stated below however, after Early Access release, our main efforts would be focused on getting Mac OS X and Linux Clients released, so if worse comes to pass and it doesn't get resolved before that point, that's when it would definitely be fixed.