What did Hashem remember?
Rashi: His Kadosh word with Avraham; He promised "v'Acharei Chen Yetz'u bi'Rchush Gadol 1 ; v'Dor Revi'i Yashuvu Henah" (Bereishis 15:14, 16).
Radak: He did this for them in the merit of Avraham. They were not worthy of it, for they quarreled and tested Hashem. Also to leave Egypt they were not worthy, if not for the Bris with the Avos - "va'Yizkor Elokim Es Briso Es Avraham
Malbim: Hashem asked Moshe to ask Yisrael to request gold and silver vessels from Egypt, lest Avraham have a complaint that the prophecy of affliction was fulfilled, but the prophecy of leaving with great wealth was not fulfilled (Brachos 9b). Also the cloud, water and manna were in the merit of Avraham (Bava Metzi'a 86b).
