Actually it's contested that Bulger was involved in drugs-for-weapons exchanges. Quite a few have pointed out that Bulger didn't bring in any shipments himself. The money he made of the drug trade was because he taxed dealers that were doing business in South Boston.

As for the IRA connections; an IRA source claimed that most of the heavy lifting in the arms trade was done by guys like Pat Nee, who were actually off-the-boat Irish. And contrary to popular belief, Pat Nee was not part of Bulger's crew. Later on he did have dealings with Bulger, but as up-and-comers both made quite a few attempts on each other's lives and there was always a major sense of distrust between Bulger and the guys that were born in Ireland and were involved in the IRA weapons trade.

When it comes to the gang he ran...I can really only find info on Flemmi, Martorano and Weeks as being part of his crew... The so-called Winter Hill Gang seemed to be notoriously loose-knit.

The logical explanation I can find is that he indeed had lots of people in South Boston fawning over him and his reputation of being a legendary underworld figure and a killer. And he most definitely was an ice cold killer. So most probably just agreed to pay up just to avoid any hassle.
I just can't help but find it noteworthy there seemingly weren't any players around that were thinking that maybe Bulger wasn't as influential and untouchable as the legend around his persona made him out to be and decided to go against him in a major way.