Why did the US stop making 2 dollar bills?

The US has never stopped making $2 bills. The most recent printing has been under the 2013 series. However the denomination only makes up about 1% of all bills in circulation.

The denomination was once common but suffered from an association with "tainted" activities; it was the preferred bill for placing $2 bets on horse races as well as being the going rate in brothels in pre-inflation times.

As noted, a decline in use made the bills seem rare when they did show up in circulation causing people to hoard them, which caused even fewer to circulate, and so on. The problem has been exacerbated by the Treasury's response to the problem by simply printing more and more $1 bills, to the point where that denomination now comprises almost half of all bills in circulation.