Blame Campagnolo.

Prior three arm steel was very common for interchangeable rings, then the Simplex adaptors that provided a 6 place attachment for aluminum rings.

Alternately there was the 50.4 mm BCD 5 point attachment to the outer ring. In steel then Aluminum. Tourist bikes ended up with those, some track bikes too way back.

Campagnolo brought out the 5 arm Aluminum cotter less crank in the late 50's. Slow to catch on, it overwhelmed eventually, Surpassing the Stronglight and TA alternatives, TA having a three arm model and a 50.4 BCD.

Williams Made a nice three arm Steel Cottered crank set with nicely machined steel rings, weight was not bad either.

I think it was Racer Worry that three in aluminum was not enough. Many wanted to stick with steel cranks as they were worried about aluminum failing...