When and why did Mendelssohn's "A Midsummer Night's Dream" become the most popular musical accompaniment for wedding ceremonies?

Firstly, I wouldn't say that the Wedding March (Hochzeitsmarsch) from the incidental music that Mendelssohn wrote for A Midsummer Night's Dream is the most popular music at weddings. In fact, the Wedding March shares its popularity at weddings with the Bridal Chorus (popularly known as Here Comes the Bride) from Wagner's Lohengrin. The two pieces are complimentary. Here Comes the Bride is stately, solemn, almost ethereal and is used for the moment when the bride floats, swanlike, down the aisle on her father's arm towards the man with whom she will share her destiny while her mother wipes away a tear, etc, etc; the Wedding March is 'an ideal mixture of bombast, sentimentality and gravity' for the recessional as the laughing couple, with the crowd of guests behind them, walk out to get rice thrown into their hair. That's part of the 'why’ the March has remained popular for so long - it's great music and fits the mood at the end of the wedding very well.

The incidental music to A Midsummer Night's Dream was written in 1843 and premiered in October 1843 at Potsdam (the famous Overture had been written in 1826, while Mendelssohn was still a teenager). The British premiere, with the composer conducting, was in May 1844 - (Mendelssohn was a frequent visitor to Britain, where he was very popular and was befriended and greatly admired by Queen Victoria, the Prince Consort and the other royals). A vocal score was published in 1844, and a full score in 1848.

In March 1899, after the Wedding March had been popular for a considerable time, Samuel Reay, a British organist, wrote to the Musical Times, noting that he had transcribed the piece for organ and performed it for the first time at the grand society wedding of Mr. Tom Daniel and Miss Dorothy Carew, at St. Peter's Church, Tiverton, June 2, 1847. Whether or not Reay was the first to have played it at a wedding, no one seems to have stepped forward with a claim of a prior performance. https://books.google.com/books?id=npAPAAAAYAAJ&pg=PA194 Punch mentions that it was played at a fashionable wedding in Knightsbridge in 1857. https://books.google.com/books?id=PmlPAQAAMAAJ&pg=PA50

It is often said to have been the wedding of Victoria, the Princess Royal and daughter of Queen Victoria, to Frederick Wilhelm of Prussia in the Chapel Royal in St James’s Palace on 25 January 1858 that made the Wedding March fashionable. It is possible that this was a request made by the bride or her mother to honor their dead friend, but this is unknown. At any rate, from here on the piece was often played at weddings, and by 1870 one writer notes that “in these times, all the young ladies 'in the fashion’ expect to have Mendelssohn’s Wedding March played over them”. https://books.google.com/books?id=YLMPAAAAYAAJ&pg=PP8