Why didn't he compel Saul? Certainly He could have done so.
Yes, He could have compelled Saul. He certainly compelled Jonah. And He compelled Balaam the false prophet to bless Israel when he didn't want to.
The Bible (even the Old Testament) is often unclear why God does something. Take for example Job. God never explains to Job why He allowed Job to go through all those problems. This is part of God's sovereignty and He will not compromise it or explain it.
Personally I believe God lays down the rules and when we follow those rules we are blessed. When we do not follow those rules we have problems. Esther brought blessings on herself and the Jews just for speaking up at the right time. Jonah just sat in a stinking fish while he consider whether to speak up or not. One got a blessing for obedience. The other got body odor.