It is because following the akeida, Avraham realised that Yitzchak became very close to being killed which would have ended both Avraham's bloodline and legacy. Therefore, he started thinking through potential spouse options for Yitzchak that would meet his criteria (see Rabbeinu Bachya), which is why he starts reviewing his family now.
אחרי הדברים האלה AFTER THESE THINGS [IT WAS TOLD ABRAHAM] etc. — When he returned from Mount Moriah Abraham was pondering and he said, “If my son had really been slain, he would have died without children! I must marry him to one of the daughters of Aner or Eshcol or Mamre”. The Holy One, blessed be He, therefore had the announcement made to him that Rebecca, the one fit to be his (Isaac’s) consort, had been born. This is what is meant by “after these things or words” — namely, “after the words” that expressed the thoughts aroused by the Binding of Isaac (Genesis Rabbah 57:3).
(Also refer to the Bechor Shor)
Ibn Ezra adds when he mentions Milkah it is to hone in on Rivka's pedigree.
BEHOLD, MILKAH, SHE ALSO HATH BORNE. This is stated to inform us of Rebekah’s pedigree/lineage.
(Refer also to the Ohr HaChaim and Radak who says it is intimating that Rivka had now been born)
And if Rivka was not suitable, then Sforno mentions he was assessing other options:
She also gave birth - Avraham was told that if he did not wish to take Rivkah for his son, Re’umah also had a daughter — Ma’achoh — who was fitting to be Yitzchok’s wife.
Alternatively, if we take a more literal approach and question whether or not he actually knew them and Ramban speaks this out.
BEHOLD, MILCAH, SHE ALSO HATH BORN CHILDREN. Since Milcah was the daughter of his brother Haran, this was a tiding to Abraham that his older brother Nahor had been visited with many children from the daughter of his dead brother Haran. Now from the text of Scripture it would appear that Abraham had no knowledge of any of them except on that day. If they were visited with children in their younger days, it would be impossible for them not to have been heard until this time for the distance between Mesopotamia and the land of Canaan is not great. Now when Abraham left Haran he was seventy-five years old, and Nahor was also elderly and his wife too was not young. Indeed, we must say, G-d performed a miracle for them in that they were visited with children in their old age. This is the sense of the verse, Milcah, she also. In the words of our Rabbis, it is said that Milcah was visited with children as was her sister Sarah.