i don't know for sure, but i assume its because they feel that the computer is doing the job of stopwatch. stopwatch was put in in gauge mode for people doing manual (tables) decompression to manage their stops and slides, to manage 'time at max depth', etc..
I don't think at the time we had any feedback that people wanted stopwatch for regular dives.
for something like this, the best plan of attack is to formulate a solid reason why you want the feature added (not just 'why isn't it available') that you can send to UEMIS (via email or ticket) and i can approach Ernst with, and refer back to customers letters/emails/whatnot.