Most, not all, were built as A-1-A type flexicoils. This gave them the same axle loading as a 80-85 ton locomotive or car. Many of those Canadian branches had light rail but it wasn't politically possible to abandon them or always financially feasible to upgrade them.

The disadvantage to an A-1-A freight design, is that only 2/3 of the locomotive weight is available for adhesion. Compared to a comparable SW1200, they only had 2/3 of the starting tractive effort, but their low hp (1200) meant their continuous tractive effort was likely right around the nominal minimum 25% adhesion.

As the light rail branches were sold/abandoned/upgraded, there was no longer an operational advantage to having a fleet of locomotives with light axle loading. CN had several rebuild programs which resulted in many of them being rebuilt with salvaged B-B flexicoils. The basic electrical circuit required no changes as it was already four-motor.

Similar situation existed elsewhere. Pacific Great Eastern (BCR) bought Alco RSC2/3 units, retrucked them to B-B as track improved. Milwaukee had some Alco RSC3 and SW1s with mu to run some of their prairie branches. SP's first big Baldwins were 5200-5202 DRS-6-4-1500 (A-1-A) and two were on the Toledo branch until it was upgraded, then ran out of southern Arizona until two sold with the Nacozari line and third was scrapped. SP also had a a few 44 tons and large fleet of 70 tons (as did CN) whose use declined as light rail/bridge lines were sold/abandoned/upgraded. SP still had SW1-SW8-SW900 units until they were no longer needed. There were still a few places the SW1500s ran in deference to lighter rail, as well as the first order of SD9s (scattered in low 4300s when rebuilt) that were lighter (as were the early C-C Baldwins).

The reason that the GMD1s survived so long is that there was still a use for them, either on light rail as A-1-A or where a low hp road unit works just fine. Compare that to UP who sends out numerous locals with a pair of GP60 or SD40/60 units where one smaller reliable bidirectional unit would work fine.