As wikipedia states, the Bourne shell was written as a replacement to first shell ever, Thompson shell, and the latter was really stupid so it's unlikely that this design decision was derived from the predecessor. However, Bourne has used some of ALGOL 68's syntax rules, and this may be the cause... someone who knows ALGOL, please, correct me.
Such basic design decisions are rarely traceable to their roots just because they were obvious to original programmers, or taken from some common that times and now dead language, or just chosen randomly from a list of possibilities.
For me, the dot command associates with "the current something", i.e. like an alias to the interpreter in case of shellscript, and the short form means there are no process fork'd. Well, this is not a good explanation, but not worse than any other.
upd: You can also take a look at this article in ComputerWorld. While it does not answer your particular question, it may be interesting of itself.