What is the main reason for why January sucks for movies?

January in general is a super-boring month. Its only noteworthy holiday (if you could call it that) is New Year's Day...and then that's it. You then spend the next 44 days (6.3 weeks) enduring cold snow or just plain winter weather (depending on the place you live in) and doing business as usual until Valentine's Day rolls around. It gets easier when February starts up, because people get excited about the next holiday, but still, you gotta deal with the boring everyday slog until then.

It's only fun for some people if, say, they had their birthday during that time. Then the occasion could brighten up the spirits of the family. Plus you get a garnet for a birthstone.

Weather and lack of funds after holiday spending keeps folks at home.

far from the awards season.

You've got it close. Studios have been loading up their release calendars for decades now such that October through December typically contain a disproportionate amount of mass-audience and/or awards contender pictures compared to the rest of the year. The post-holiday period of January through early February is a known slow box office period, and most viewers during that time are still taking in movies that premiered over the holiday period.

Distributors also find it a lower risk to release movies that would have otherwise been complete duds outside of more competitive periods. Bad Boys 3 probably wouldn't have set the world on fire had it been released over Memorial Day or the Fourth of July weekends. Just as well, a studio might not have the utmost confidence in a movie (Dolittle being a recent example), so the doldrums of January serve as a suitable time frame to fulfill the contractually-obligated theatrical release where it stands a better chance of recouping some revenue rather than being an out-and-out loss.

As long as audience viewing habits remain, this will likely remain the norm.

Many of the big awards contenders go into wide release early in the year - I tend to see more films in January/February than any other time of the year.

New releases in January do tend to be junk, but those are avoidable.

I believe it’s because of audiences have spent too much money during the holidays and vacations so in January people try go less to the movies because they are trying to recover for spending too much in December.

And studios know that so they take their worst films during that season to not loose that much.