As a former subcontractor that was forced to use Deltek for a couple years, I will agree that it's the most popular... but I haven't the faintest notion of why. It wasn't the worst option available, but that didn't stop it from being terrible regardless.

Time-tracking keeps getting reinvented because no one's solved it definitively yet. I don't think anyone ever will, either; time tracking relies too much on individual context for it to be wholly standardized.