Because God has decreed that His purposes will be accomplished (exactly how, when and where He wants them to occur) through the free choices His people make, of which He has decreed both the choice, and the result.
Thus: why did Wesley believe wrongly about the Gospel? Because he was left to his own devices to study and seek out the truth, and like all of us he was not perfect.
However, equally true at the same time, and with no contradiction, God decreed that Wesley would get it wrong. Perhaps Wesley's incorrect interpretation of the Gospel, served as a balancing force to keep Hyper Calvinism in check. Or perhaps it was to keep God's church from flooding the world before the proper time. Who knows? The point is, that God both allowed it, and decreed it, and His purposes are His own. John Calvin said, rightly I believe, that where scripture is silent, we should be silent. Thus, to posit answers (whose possibilities are in the billions, surely), is a ridiculous waste of time and energy.