Is there any good reason why so many miners are apparently mining it?
They must have an incentive. I can't really believe that paying the hosting of a faucet website plus the mining costs are less than the ad revenue, but they must have a profit somehow, otherwise they wouldn't mine. That profit may not be translated into money.
Or maybe it just has a different difficulty target / other parameters that were chosen?
It has the same difficulty parameters. Check
chainparams.cpp#L180.