My take on what happened (assuming this is actually how it ends):
Fatty being this much of a violent sped, culminating with him comitting battery against Letty (the spitting), made Sam/Jet panick, deciding he needs to go NOW as hes too much of a legal risk.
They cant just kick him, as they have the gofundme linked right above the chat and that makes them look weird, plus they would be basically just like idubbbz in the public view (cutting Fatty off). Plus the compounding costs of keeping the site up and the repairs from everyone trashing the house (Sam says at least $60k damage, i believe it tbh) makes them just say fuck it, we'll call it early.
Plus even if it went the full 6 weeks, i dont think they actually had an ending planned out, judging from how the last few weeks were hamdled.
Also imo Letty deserved the grand prize, she by far took the most shit out of everyone there, was the only one that actually got a Hell House

experience, and took it like a champ.