How and why do sequel writers get away with blatantly wrecking previous films?

It's been a problem since movies were invented, sadly. Usually it's due to bad writers/directors/producers having friends in high places. They only partially liked the previous film, but felt they had to "fix" whatever they didn't like in the previous story, and it turns out they are a talentless hack that couldn't do anything original to save their lives. There's a reason they call it "sequel syndrome," where you see a wonderful film, and then it has a crappy sequel with all the worst tropes of bad sequels.

It is unfortunately a situation where the aspects of fanfiction.net come to life on the big screen, and you wish these morons had just gone to fanfiction.net to write their crap instead of wasting millions of dollars just to torture us for two hours in the theater with their garbage.

Typically, if a sequel really sucks, people will see it once and never again, or even acknowledge that it exists. It's particularly common in movie franchises, where at least one or two films were so bad that the fans will pretend it never existed, and if they're lucky, the people in charge of the franchise will honor their wishes and never bring such stories up again unless asked.

SPEAKING ON SLASHER FILMS...THE STUDIOS ARE BEHIND 90% OF WHAT YOU GET...THEY ARE THE ENTITY PUSHING PROPERTIES FORWARD PAST A COUPLE FILMS AND INTO FRANCHISE TERRITORY.