Okay, you could say it's semantics, but it wasn't abandoned. Gumdrop still exists, he still makes money on the ghost of his game.
It's being held for ransom. And just like every handbook out there says you don't negotiate with terrorists, people got suckered into negotiating with the terrorist Gumdrop.
The net result is that it's just like being abandoned, only you have some hope that it will resume at some point.
Reason? It doesn't need any reason. If you need an actual reason, pick anything: greed, laziness, incompetence. If Gumdrop was able to hire an artist/coder/copywriter in the past, he could do it again. If he was doing it on his own, he could keep going.
Simply put, there's no actual reason for the game not to continue.
My recent theory is that Gumdrop was in financial trouble, sold his Patreon campaign, copyright to the game and story and rights to his assets and promised to keep his mouth shut. When the new owner saw he was losing patrons, he commissioned Gumdrop to make the Christmas special. After that, he may or may not have contacted Gumdrop again to make Part X, but could have shopped around for new artists, coders and writers to finish the game.
The only snag is that anybody willing to buy a Patreon campaign would want to get the most out of it and would quite likely be competent enough to hire people to run and complete the project.
Meanwhile, the original Gumdrop started another project, maybe even finished it, and wants nothing to do with Dual Family ever again.
