Emulsion changes are certainly part of it. Just within films I've used in the past year or so, I see close to 2:1 variation in development time for the same nominal film speed -- Forte 400 requires close to twice the time as TMY with the same developer, temperature, and agitation regimen.
One contributing factor, within "emulsion changes", is grain -- modern films are generally much faster for their grain sizer (or have finer grain for their speed, same thing) compared to those from 50-60 years ago; even the "old school" films we get now, like Forte and Efke, were the "new, thin-emulsion" technology of the 1950s, and genuinely modern films like T-Max and current formula Tri-X are two generations newer than that. And the smaller the halide grain, the less development it requires, because it takes a certain amount of time for the halide solvent (sodium sulfite, in D-76 and most other conventional fine grain developers) to remove enough of the halide grain surface to develop internal latent image specks and produce full film speed.
So, bottom line, the finer the film grain, the less time it needs in developer -- and some modern ISO 400 films are comparable to the (old scale, multiply 2x to compare with modern ratings) ASA 25 films of the 1940s in terms of grain.
