Two reasons for not providing slot times off the top of my head.
First and foremost, at alomost all times of the day that we run delays the gates space that you have vacated is needed for an inbound. We try to stage ac is the correct order in a position to shut engines and wait as much of the taxi delay as possible parked and running of the apu.
Second reason from a traffic management standpoint unless every ac would accept a slot time and for the aforementioned reason they won't, it would prove to an impossible task to manage the ground sequence and reconstruct the order from many different places. (ramps, taxi line, off ramp remote parking) The taxi order or departure sequence can mean the differance between 15 minute delays if constructed well and 45 min. delays if constructed poorly.