Why didn't DC let WATCHMEN go out of print?
Because it sells.
Because it sells? Or because sometimes a low seller will keep the rights so it can be sold again later in higher numbers?
Every few years, Marvel will release (for instance) a Dr. Strange miniseries. Not because they expect huge sales, but because if they don't, a few key trademarks will expire, and in the long run, it's worth it. I would bet hard money that the same is true for Watchmen.
But the reason I say it's neither here nor there is:
1) Because I'm not in a DC boardroom; this is conjecture on my part, and I don't like to base an argument on conjecture.
2) Because I really don't think it matters. "We promised you something, but then we realized we could make more money if we didn't do that, so... y'know. Go fuck yourself" is enough reason for me to say "that's shitty."
In 1986, Moore was 33 years old, and he already had written a number of projects for DC. I hate to say it, but if that point was so important to him - "WATCHMEN will revert to me and Dave by XX date" - he should have gotten it in writing.
Jack Kirby was 44 when he did Fantastic Four #1,
and had drawn his first professional superhero comic 21 years earlier. Was he wrong to be pissed off?