rostasi

In the 60s, the only sandwich they served was roast beef because they wanted
to be a fancy version of burger joints. Even their prices were way over the top.
Commercials often emphasized the "R" and the "B" and the love of "tasty
roast beef" (they used to call it "the 4-hour sandwich"). Later, there were
roast beef variations (with cheese, etc). When it was clear that that approach
wasn't working, they started adding other things in the 70s to fill out the menu
with some variety. All in all, it took a massive nosedive later even after giving
in to the temptation of having the items that other fast food joints were offering.

So, the fallback on the idea that it was named after the brothers is a bit
disingenuous since they rode that roast beef pony pretty hard during the early days.