The Spark's processor is already maxed out -- I don't think it could meet the additional demands of APAS. And the sensors probably aren't sophisticated enough. But only a DJI engineer could say for sure.
From a marketing standpoint I don't think it makes sense either. Giving the current Spark APAS with a firmware would steal sales from the more lucrative Mavic Air.
They could add APAS to a Spark Pro or Spark 2.0 but our "feature wish list" is getting impractical at the current price. They would need to raise the price substantially or trim some features - not sure if APAS is as desirable as 4K, 3-axis gimbal, bigger battery, etc.