It started as a moba. Ironically it was removing that element that killed support for the game in the end. They didn't release an offline standalone because I'm pretty sure they laid off the dev team.

It had the wrong financial model. It was originally intended as a live service competitive MOBA - a pivot away from the traditional OMD model. They (Robot entertainment) pivoted back to a more traditional OMD game but didn't change the financial model and live-service set up.

It was free to play but instead relied on cosmetics purchases & micro-transactions - this model works for competitive PvP live service social games (such as MOBAs) but not Co-Op and PvE games (such as traditional OMD games). And so there wasn't enough money to cover all the expensive ongoing costs that come with a live service game such as servers, customer support, E-sports and other Twitch influencers etc. I still remember they used to have regular streams of "OMDU TV" with influencers and hosts etc trying to drum up exciting content that was clearly meant for a competitive game.

Anyway, if they had of just charged $40 to buy the game outright and ditched the on-going live service model it would have been perfectly fine.