In the beginning it was Western style cafes serving milk tea. They had to create special prestige using a tea more exotic than Chinese or Indian. Therefore Ceylon was the better choice. Ordinary grade Ceylon tea was much sharper and more depth than ordinary Indian tea, such as Assam BOP, that made a stronger reason. Because after ading milk and sugar there is still Ceylon tea taste. Also aroma.
Vietnamese tea is use now to blend in not for better taste, but for cheaper price. Good cafe still use only Ceylon tea.
