Bell made more money selling Norstar for small sites where Centrex EBS would have worked, but they weren't that bright. NT essentially had to tell them what to buy. Since they owned a good chunk of Northern Telecom, they both made more selling Norstar. NT pushed equipment sales to boost its own revenues whereas Centrex would only boost Bell's.
Norstar was a deception. It was a dumbed down version of SL-1/Meridian-1, which was much easier to set up. Lots of installers get scared of MICS and Option 11-81. Put them on DMS, and they pass out.
Whereas I liked BCM, I never liked Norstar -- the first version of it was simple, fine. That feature code BS, meant for analog set support, was a step backwards I felt. They called it a key system. Key systems by definition cannot switch a call from the CO to any station. Norstar time-switched calls. It was a switch that simply behaved like a key system (mostly) in its initial release. Everyone would pick the square template and I'd moan. When they got into 8x24 and target lines, PRI, BRI in CICS and MICS I really thought it should have just gone to an OPTION 11C MINI. Then it would be LD this and LD that. They'd be lost. Two systems with incompatible phones doing the same thing.
Bell using Norstar for internal reasons was silly, because they owned DMS-100s that were licensed for Centrex (Microlink ISDN Centrex for example could do Electronic Key Telephone Service (EKTS) and CACH - Call Appearance CALL Handling) -- I always say cache by mistake. The earlier version of "p-phone" Centrex (EBS) had all the same features. The only reason I could see Bell using Norstar itself for small offices was on a CICS or MICS where "lines" from DMS were limited, or where they had to buy DMS line cards for Microlink Centrex or EBS Centrex or what not. What each went for back then I don't recall. I doubt Norstar would have come out cheaper. Bell just wasn't that smart, and ISDN was much harder to set up.