From J.C. Ryle's "George Whitefield & His Ministry" as a brief bio in the front of Select Sermons of George Whitefield (also found here), we get a picture of his open-air preaching and the church culture that pushed him toward it (bold in the text is mine)...
Two months after this Whitefield began the practice of open-air preaching in London, on April 27, 1739. The circumstances under which this happened were curious. He had gone to Islington to preach for the vicar, his friend Mr. Stonehouse. In the midst of the prayer the churchwardens came to him and demanded his licence for preaching in the diocese of London. Whitefield, of course, had not got this licence any more than any clergyman not regularly officiating in the diocese has at this day. The upshot of the matter was, that being forbidden by the churchwardens to preach in the pulpit, he went outside after the communion-service, and preached in the churchyard. ‘And,’ says he, ‘God was pleased to assist me in preaching, and so wonderfully to affect the hearers, that I believe we could have gone singing hymns to prison. Let not the adversaries say, I have thrust myself out of their synagogues. No; they have thrust me out.’
From that day forward he became a constant field-preacher, whenever weather and the season of the year made it possible. Two days afterwards on Sunday, April 29th, he records: ‘I preached in Moorfields to an exceeding great multitude. Being weakened by my morning’s preaching, I refreshed myself in the afternoon by a little sleep, and at five went and preached at Kennington Common, about two miles from London, when no less that thirty thousand people were supposed to be present.’ Henceforth, wherever there were large open spaces round London, wherever there were large bands of idle, godless, Sabbath-breaking people gathered together, in Hackney Fields, Mary-le-bonne Fields, May Fair, Smithfield, Blackheath, Moorfields, and Kennington Common, there went Whitefield and lifted up his voice for Christ.
FOOTNOTE: The reader will remember that all this happened when London was comparatively a small place. Most of the open places where Whitefield preached are now covered with buildings. Kennington Oval and Blackheath alone remain open at this day.
The gospel so proclaimed was listened to and greedily received by hundreds who never dreamed of going to a place of worship. The cause of pure religion was advanced, and souls were plucked from the hand of Satan, like brands from the burning. But it was going much too fast for the Church of those days. The clergy, with a few honourable exceptions, refused entirely to countenance this strange preacher. In the true spirit of the dog in the manger, they neither liked to go after the semi-heathen masses of population themselves, nor liked any one else to do the work for them. The consequence was, that the ministrations of Whitefield in the pulpits of the Church of England from this time almost entirely ceased. He loved the Church in which he had been ordained; he gloried in her Articles; he used her Prayer-book with pleasure. But the Church did not love him, and so lost the use of his services. The plain truth is, that the Church of England of that day was not ready for a man like Whitefield. The Church was too much asleep to understand him, and was vexed at a man who would not keep still and let the devil alone.
