Aside from the obvious problem pointed out by
@baldgye already, there's the small issue of determining what 'Soft' and 'Hard' Brexit actually mean - there are potentially any number of definitions of 'Soft Brexit' for example, and the most obvious one (that seems to be most popular even with Leave voters) is the infamous 'cake-and-eat-it' model - i.e. ditch all the bits of EU membership we don't like and keep all the bits we do, which the EU itself has categorically ruled out from before the referendum was even held, so there was (and still is) not much point in putting that option to a public vote in the UK!
Similarly, it isn't even clear what 'Hard Brexit' is - it seems to range from everything from a new customs union to no deal whatsoever, so again it wouldn't make the process any clearer.
On the flip side, the 'Remain' options could have been split into at least two camps as well - 'Remain and push for ever closer union', 'Remain and reform' (a popular one!) or 'Remain but strip back the EU to a simple trading bloc'.
As such, a simple 'Remain' or 'Leave' was the only sensible option for a referendum question, and the result should be used to figure out (via Parliament) what the options are from that fixed starting point.