Handel's "Messiah" was actually first premiered 13th April 1742 at the Great Music Hall, Fishamble Street, Dublin, Ireland. In early March 1742 Handel began discussions with the appropriate committees for a charity concert, in aid of prisoners' debt relief, the Mercer's Hospital, and the Charitable Infirmary, at which he intended to present Messiah. It's highly plausible that he may have treated the charity performance in Dublin as an equivalent to the modern nearly finalized work-in-progress Theatrical Performance "Preview"s where the show is adjusted before its official opening. In this case, the later London performances would have had the polished finalized version after any adjusting to the piece, once it was being publically performed.
It's unclear, from this program, if Handel could only get Messiah performed if it was in aid of Charity as it was seemingly poorly received as a standalone piece otherwise, or if he decided to turn it into his go-to Charitable performance piece due to its suitability for Christian Charity fundraisers.