They'll do 75 fine, but it takes a little while to go faster, top speed being under 100mph. Singles don't have high redlines.
GB500, MZ Scorpian, Yamaha SR500, Buell Blast were the main singles of the last 25 years, and they just don't sell. The MZ used a Yamaha 660 engine and was a great bike. The SR handles great, and has terrific bottom end (and makes a great cafe racer)- it was kick-start only, but still...
The GB is a purpose-built bike, and frankly is uncomfortable for many people on long rides. It's not for touring. The bars are low and it has real rear-sets; some folks complain about their wrists, and some about their hips/knees. Also, a lot of people were irritated that it was a blatant imitation of an amalgam of British singles with "TT" on the side. In other words; blibbity blop. Smaller bikes are appreciated in countries where riding skill is important, yet borders are closer together. Here in the states, there are roads you can't see the end of and "we've" been raised on "bigger is better". It doesn't take any skill to navigate a 6 or 8 lane freeway.
Or, smaller bikes are considered beginner or girl's bikes. People used to hammers have a hard time negotiating a scalpel.