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Its entirely marketing, both Hasbro's success,a and Tonka's failing.
In short, the mainappeal of machine robo were that they were elegant, small toys that were cheap to buy and easy for kids to keep in their pocket; basically like Hot Wheels or Matchbox cars. There were some larger figures, but not as large as what Hwsbro would brand for Transformers. These toys were designed by PLEX(the sentai/Kamen Rider design team, among a lot more) and were well loved among kids.
Hasbro preyed into American mentality that bigger=better, and shaming people into being concerned with buying "Real" Transformers(similar to how they'd try to muscle out non-JP Dinosaur toys). Now non-Transforming lines weren't just competitors, they were imitators.
That,and Tonka stuffed the US line with very cheap, poorly engineered bigger figures, and commissioned a very poor cartoon that wasn't as cool as Transformers.
The effects of this have been unfortunate, with Hasbro continuing to dominate transforming robot toys in the US(Power Rangers being the closest competition before they bought it; and historically the US side of that franchise alwauys focused on ranger toys over mecha). Hasbro marketed Transformers so well in the 80's anything else is seen as a ripoff now. Hasbro continued trying to keep MR from being imported, buying up Tonka, and the rights to the Machine Robo Rescue name in the 2000's.
