I think when the decisions were made, the 100 seat market was not very viable. Scope did not allow regionals to fly airplanes that large, and mainline pilots probably wanted too much money.
By the time the majors were able to cut pay for 90+ seaters at mainline, the 717 was somewhat older technology.
Also boeing wanted to cut overhead, and since the MD stuff had no commonality with boeing products, they didn't want to keep that division going just for one small airplane.
Airliners are pretty much all capable of flying through the same Wx, if your flight got canceled it was probably for the airline's own operational reasons (airplane broken, late, or limited arrival slots). The type of aircraft would not matter in the 121 turbojet world.
Did you know Terry Beezhold?