what could be the reason my 1999 ford explorer want hold a charge after buying a new alternator and same battery but autozone say the battery good

My daughter's 99 explorer had the same problem. Battery and alternator both good. finally traced it down to a stuck switch on the drivers door that allowed the interior lights to stay on when door was closed. The switch is inside the door so the panel has to be removed. The space is tight and removal is difficult. Soaked the switch in place with electrical contact cleaner and WD 40 and it freed itself up. Try that first if the lights stay on. As JS said, 3.5 to 4 years is about all you're going to get out of a battery. The weakness will show up with the forst cold snap. Also check all connections on a 15 year old vehicle for corrosion.

The inside lights go off when door is closed