From their updates it looks like scope creep starting in 2021 and a lack of leadership.

In 2021

> Even so, we know that half a year is a long time to wait for an update. The reason that answers have not been more forthcoming this year is because, until recently, we did not have them.

2022

> we’ve made the decision to redevelop Hytale’s engine—both the client and the server—in C++

Winter 2023

> This year, we've brought engine development fully in-house and steadily expanded our team

2024

> As we mentioned in our Winter Update, the first half of 2024 has been focused on finishing that process and transitioning our creative teams into the new engine.

They're still transitioning to the new engine.

Summer 2024 future plans

> The rest of the year [...] the goal [...] is to verify that our engine, tools and processes are capable of supporting our gameplay and creative goals.

Ten years of development with 40 employees is basically a guarantee of failure. A lot of the time, putting more people and money into the team makes it less likely to succeed because all those extra resources need to be doing something. You start off with a pretty simple plan and then the scope goes out of control and you burn cash before crashing into the ground.