There's no two ways about it that the CB750 was not the first but to answer Mos question,

the answer is I dont know.

Some road going bikes from days gone by had small cast iron drums that would strugle to stop a modern day 125.
Then look at some of the double sided twin leading shoe aluminum and magnesim brakes of the early 60s race bikes. The only reason that I could see is that they would be lighter than two big cast iron discs.
A lot of people tried discs for racing in the 60s and reverted back to drums. The early cable operated discs were made by Campagnolia as probably were the ones fitted to the Lambretta and a lot of racers found they had to pull on the leaver for all they were worth in an efort to make them pull up.
In the mid 60s, Farhon my Sponsor of today started producing TLS racing brakes and saw off this new disc brake craze, if you didn't have one of Farhons brakes you were nobody.
Perhaps with the introduction of hydrolics the disc started to make a name for itself.
Who knows what the answer is, it's probably out there in Internet land waiting to be found.
Sam.
