But while he thought about these things, behold, an angel of the Lord appeared to him in a dream…
Matthew 1:20

The Lord dispatched Gabriel to tell Mary about her miraculous pregnancy before she conceived. But God didn’t say anything to Joseph until after (1) Mary was “found with child of the Holy Spirit” and (2) Joseph had to wrestle with what to do about his pregnant fiancé. Why did God tell Mary before the fact and not Joseph until considerably later?

I cannot answer for God, and since His Word doesn’t address this question, I won’t speculate. But consider:

God doesn’t have to let us in on what He is doing. That He does (when He does) is grace. As for the timing of God’s communiqués, that too is entirely up to Him.
When we do have to wait, and sometimes struggle with what is happening in our lives, waiting teaches us patience—a fruit of the Spirit. Joseph’s struggles revealed his good and kind heart in not wanting to make a public example of Mary.
In addition to the opportunity to grow in patience and see what is in our hearts, not knowing what God is up to affords us an opportunity to trust God, and give Him what He prizes in us above all things: faith (Heb. 11:6).

Joseph had to wait, but in waiting, the sovereignty of God was displayed, the heart of Joseph was revealed, and the man God chose to raise His Son grew in faith.