TAS won an Emmy and everyone seemed happy with it. So why did it go off the air after only 2 seasons? Ratings?
Economics.
The way most cartoon series were contracted in the 70s and 80s, esp. those TV spin-offs (paying big royalties to the creators of a parent live-action series) very, very rarely went beyond one or two seasons. The first year was a typical-length season, the second year was a half season padded out with repeats.
It had been discovered that kids watched episodes of cartoons
whether they'd seen them or not. So even episodes that had been played tens of times got the same ratings as brand new ones. Think about a typical Saturday morning in the 60s and 70s: you got out of bed at about 6am and turned on the TV. If a repeat cartoon came on, you didn't go back to bed,
you just watched it again till the next one came on.
Had the series gone to a third year, all the contract renewals would have exhausted the budget, which was already so minimum they couldn't afford Walter Koenig, and until Nimoy held out, they almost weren't going to have Nichelle Nichols or George Takei either!
Also, I seem to recall the Emmy nomination was announced
after the show had been canceled?