Could be a multitude of reasons, but maybe they needed to do a next call for your departure instructions, or they were waiting for a release from departures. Or they could just be slow. Hard to hang a hat on a reason, could be a mix of all of the above. 5nm I would suggest would almost always be a gap, but there could be wake turbulence, or some additional spacing required on the same SID due to varying aircraft speeds. Not really familiar with ML ops so can only give a couple of general reasons. There isn't a reason why you can't line multiple aircraft up, though if you have arrivals as well, that's a consideration. Nothing quite as exciting as having a few lined up to depart 03 at PH and having someone pop out of JT and straight onto final for 03.