As a result of Pesach taking two Shabbatot in the diaspora, the weekly readings are out of sync with those in Israel. The readings come together again at Parshat Matot/Masei. Why was this Shabbat chosen and not early pairings between Pesach and Shavuot?
My good friend Rav Prof. Neriah Gutel recently published an article on the topic. He explains that being that the accepted division of parshat hashavua, where we finish the Torah annually, is according to the custom of Bavel/Babylon (as opposed to the original custom of Eretz Yisrael where the division was to finish the Torah every third year, Megilla 29b), it would be illogical to “force” the communities outside of Israel to combine 2 parshiot just to “catch up” to Israel, when the chutz laAretz division is the one which happens to be the more original! Only when we reach Matot-Masei, where the custom abroad is to combine them to enable the reading of parshat Dvarim on Shabbat Chazon, right before Tisha B’av, that’s when the “catching up” is necessary, so that’s when it’s done (thus exactly keeping the original Babylonian division and pairing). Kol Tuv, Rav Ari Shvat