Perhaps its simply a case of technological inertia: the various
instruments (& groups of instruments) were developed at different times
by different people; & the challenge involved in getting a whole world
of fractious musicians to agree on a single key (apart from the negative
option of whichever one the viola _isn't_ in) has proved insoluable.

After all: people like Harry Partch thought we needed _more_ keys rather
than less.  Clearly he locked himself out of carriages a lot....

All the best,
Robert Clements <[log in to unmask]>