Because in cinema, it's common to change lenses within a shoot while preserving identical exposure. This is rarely important in still photography (and even less so with the flexibility of digital).

You might say But t-stops are more accurate, allowing me to be more precise! — and that's basically right, but the main thing is that precision is overrated in photography. Generally, if the difference is under a stop, it's easy to compensate without much consequence, and if it's under a third of a stop (a typical variation in t-stops), it's barely noticeable — but if you saw that change in a film, it might jump out at you.

Presumably the transition of cinema from film to digital also makes this less important, since it's easier to fix it all up in post. (That's the answer to everything, right?) But that's not an area I really know anything about so I'm just speculating.

Of course, transmission is not everything. Aperture also influences depth of field, too — although I would argue that this doesn't matter too much in terms of labeling either, for basically the same reason of precision: subject (and focus) distance, print viewing size, and all the factors which affect depth of field mean that just knowing the f-number doesn't give you more than a general idea of results. (And that's not even considering that numbers are often rounded.)