Part of God's sovereign plan is that we learn faith, childlike reliance, godly desiring, gratitude, appreciation of his goodness, delight in his gifts, and confidence in his care... all through prayer. To this end, he may sovereignly withhold some good things until we pray for them, as part of his training of us.
That's the short version. The longer version, which I hope I've summarized correctly, can be found in Calvin's Institutes, Ch. XX, 1-3. Highly recommended on this topic.