The original Kollel system in Europe required the scholar leave his family for years so he would not be disturbed in his learning. according to the biography of Rav Moshe Landynski who became rosh yeshiva in Radin before being replaced with Rav Naftoli Trop.
https://en.wikipedia.org/wiki/Kollel
The first kollel – in the modern sense of the term – in the Jewish diaspora was the Kovno Kollel ("Kolel Perushim"[2]) founded in Kovno (Kaunas, Lithuania) in 1877.[3][4] It was founded by Rabbi Yisrael Salanter[5] and directed by Rabbi Isaac Blaser. The ten students enrolled were required to separate from their families, except for the Sabbath, and devote themselves to studying for the Rabbinate. There was a four-year limit on one's membership in the kollel.
The question is why the mitzva of marriage and family were displaced?