There may be many answers to this question. One could be for very practical reasons of
"a workman is worthy of his hire" (Luke 10:7) and those that do not work, but
are lazy will not prosper (Proverbs 6:6-11). So if the non-believer is working hard, and
the believer isnt, naturally the non-believer will seem more "prosperous"
materially.
David in Psalm 37:35, 36 says, "I have seen a wicked and ruthless man flourishing
like a green tree in its native soil, but he soon passed away and was no more..."
Then in Psalm 73 much bemoaning of the "success" of the wicked is mentioned. A
commentary on the book of Psalms by the great theologian, C.H. Spurgeon, says:
"(regarding vs. 3)... It is a pitiful thing that an heir of heaven should have
to confess I was envious, but worse still that he should have to put it,
I was envious at the foolish. Yet this acknowledgment is, we fear, due from
most of us. When I saw the prosperity of the wicked. His eye was fixed too
much on one thing; he saw their present, and forgot their future, saw their outward
display, and overlooked their souls discomfort...Yet some poor afflicted saint has
been sorely tempted to grudge the ungodly sinner his temporary plenty. (Regarding vs.
17)...Until I went into the sanctuary of God. His mind entered the eternity
where God dwells as in a holy place, he left the things of sense for the things invisible,
his heart gazed within the veil, he stood where the thrice holy God stands. Thus, he
shifts his point of view and apparent disorder resolved itself into harmony...then I
understood their end. He had seen too little to be able to judge; a wider view
changed his judgment; he saw with his minds enlightened eye the future of the
wicked, and his soul was in debate no longer as to the happiness of their condition. No
envy gnaws now at his heart, but a holy horror both of their impending doom, and of their
present guilt, fills his soul."
This is not to say that the righteous are not promised "prosperity."
Joshua 1:8 says, "...you will be prosperous and successful." But lets read
the beginning of the verse. There is something this prosperity is contingent on. "Do
not let this Book of the Law depart from your mouth; meditate on it day and night, so that
you may be careful to do everything written in it. Then you will be prosperous and
successful." We are to walk in obedience to God and His Word for this prosperity to
be available to us. But we are also to focus on Him. Luke 12:31 says, "Seek ye first
the kingdom of God, and all of these things (read vss. 22-30) shall be added unto
you." One difficulty we tend to struggle with too is "what is prosperity?"
Is it more money than we can imagine? Is it a mansion? Is it just enough to pay our bills?
Is it health? God does promise that our "needs" will be met (Philippians 4:19);
but again obedience is a key factor (Read the entire fourth chapter. Who is Paul talking
to? He is commending the faithful brethren.).
There are many scriptures that speak of our "suffering" for Christs
sake (Luke 9:23; II Corinthians 1:5, 7; Philippians 1:29; 3:10; Hebrews 2:10; I Peter
4:13).
What it seems to boil down to is if you are finding that your "needs" are not
being met, take physical and spiritual inventory ("Lord, show me if there is any area
of my life where I am not doing the very best I can for you at this point in time.").
Then, keep your eyes on Jesus, and not on the "riches of the wicked," at least
not in an envious way. Pray for their souls, because you know they will take nothing with
them when they die, and the poorest of souls that know the Lord Jesus Christ will have an
eternity of health, wholeness, joy, and prosperity in the presence of their King Jesus.