For the majority, being in the middle may be best. A few people can handle wealth properly, staying humble, and trusting the Lord. It makes sense that those with the gift of giving would have the ability to earn great wealth, so that they can support missionaries and such.
From everyone who has been given much, much will be demanded. And from the one trusted with much, much more will be expected. Luke 12:48b NCV
So some Christians should be rich, so they can use the resources to get the job done. Not all Christians are capable of properly handling wealth. They are not yet fully mature spiritually, and wealth would tempt them. Those granted wealth have a greater responsibility. Much more will be expected of them.