I know there are other options, just curious if anyone knew why the limited range.
If one made it thinner, it would lose its impact, it would be only more difficult to read at least in small sizes.
There exists simple typefaces which are rounded and still readable in small sizes, for ex. Futura Round, but its rounding is very slight when compared to your example.
The loss of impact can be demonstrated:
On the top there's proper Helvetica Rounded Bold.
In the middle is my inferior imitation what it would be as thinned, but having the same fully rounded endings.
The bottom version is genuine Helvetica Light. If we could forget its perfect forms, it would be quite the same as my thinned bold.
The nonexistence of lighter weights is designer's decision and my text, of course, is only an opinion or more likely, a guess. Linotype doesn't credit any individual person due the making of Helvetica Rounded, they only tell that it's created in their design studio in 1978.