This is a serious question and not one to be answered flippantly. It is also a question none of us is fully qualified to answer because we don't have God's knowledge or wisdom, and so we cannot understand all of His ways. Here are some things that we can observe and conclude:
1. Physical healing is not our greatest need. Life in this body in this world is a passing shadow, a vapor. Eternity lies before us, and forgiveness of sins is so much more important than being able to walk during this life. Jesus showed us this truth when four friends tore through the roof and let down a paralyzed man in front of Jesus. Jesus' first words addressed the man's greatest need: "Son, your sins are forgiven." (See Matthew 9:1-9)
2. We learn a lot and grow from our physical limitations. Weakness, sickness and disability teach us about our limitations and help us to look to God. Paul pleaded with God three times for the thorn in his flesh to be removed, but God said no. God told Paul, Paul concluded, "Therefore I will boast all the more gladly of my weaknesses, so that the power of Christ may rest upon me."
3. If God did heal everyone, we would just take it for granted. We would no longer see it as wonderful and miraculous; it would be an entitlement. We would also lose the opportunity to show compassion and to serve one another in our need.
4. One day, God will heal everyone who trusts in Him, but only after our sin is removed. The ways in which we need thorns in the flesh are tied to our sinful, fallen state in this world. Once sin is removed and we are in our resurrected bodies on the new earth, we will not suffer weakness anymore.
"Then I saw a new heaven and a new earth, for the first heaven and the first earth had passed away, and the sea was no more. And I saw the holy city, new Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband. And I heard a loud voice from the throne saying, “Behold, the dwelling place of God is with man. He will dwell with them, and they will be his people, and God himself will be with them as their God. He will wipe away every tear from their eyes, and death shall be no more, neither shall there be mourning, nor crying, nor pain anymore, for the former things have passed away.” - Revelation 21:1-4, ESV
Prayer Based on Proverbs 4:1-9:
Heavenly Father, open our ears that we may hear Your instruction,
and make us attentive, that we may gain insight,
for You give us good precepts;
let us not forsake Your teaching.
Let our hearts hold fast Your words;
may we keep Your commandments by Your grace,
clinging to Christ, our perfect righteousness, that we may live in Him.
Give us wisdom; grant us insight;
let us not forget, and do not let us turn away from the words of Your mouth.
Teach us the deep and rich value of wisdom.
May we long for, seek and find true wisdom and insight.
For Your wisdom on our head is a graceful garland,
bestowing on us a beautiful crown.