I understand the difference between f-stop and t-stop, but why are cinema lenses measured in one, and dSLR lenses measured in another?
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.)
To add to mattdm's great answer:
In addition to the added exposure precision, which is not important to photography, T-Stops are LESS precise in other ways which ARE important to still photography.
F-stop is the literal proportion of the aperture to focal length. T-stop adjusts this for exposure, but this raw value is important to depth of field. Depth of field won't change when your glass is slightly more or less dim. That makes the T Values a little misleading when calculating depth of field.
To further emphasis how little a difference T-stops make in photography, consider that your frame of reference will be based on other lenses, none of which are perfect, so the difference you will get is not the full difference between the T and F stop values, but the differences in actual T values between lenses. Differences between lenses of similar price, quality, and type will be small.
F-Stops matter most when you care about knowing your composition and depth of field, T-Stops matter most when you care about knowing your exposure.
Photographers want to control composition first and adjust exposure as needed. Cinematographers need to control exposure first and then compose as needed.
The critical difference with photography is that we can choose any shutter speed we want (within reason) to get the exposure we want. In cinematography, especially in the old days of film, you had your ISO fixed by the film loaded in the camera and your shutter speed was fixed by the frame rate of the camera** so the aperture (and maybe an ND filter) is the only control you would have over the exposure. Knowing the T-Stop is critical here or your shots just won't come out right.
In photography you are creating a single image, so control over the composition and depth of field is by far more important than minor relative differences between the actual light transmission between lenses. You adjust your shutter to compensate and it's not a problem at all. What you really care about is how your photo is going to look - what is and isn't going to be in focus. Here the F-stop gives you a more accurate measure.
** Ok, but you could have a shutter speed faster than the framerate to compensate exposure, right? Yes, but shooting video with a fast shutter at cinema framerates of 24fps ends up producing jarring, stuttery motion that is awful to watch. A shutter speed near the framerate is needed to produce the natural motion blur that we associate with a pleasing cinema experience.