The Kotzker, the Divrei Chaim, and other tzaddikim zt'l explain that Rashi is hinting to us that chotmin becha, the end of galus, will be in Avraham's merit. Each of the Avos had their special qualities. Avraham Avinu excelled in chesed. Chotmin becha means that the galus will end in the merit of us going in Avraham Avinu's ways and performing chesed for Yidden.
Several parshios of the Torah discuss Avraham and Yaakov, but only one parashah (Toldos) discusses Yitzchak Avinu. Gedolim explain that this is because the main middos that Hashem wants in the world are the middos that Avraham and Yaakov excelled in, the middos of chesed and rachamim. Although Yitzchak's middah of gevurah is essential and one of the foundations of the world, the Torah elaborates more on Avraham and Yaakov, so we will know where to place their emphasis in avodas Hashem. We must be occupied in doing deeds of chesed and rachamim, to help others, as it states (Tehillim 89:3) olam chesed yibaneh, the world was built with chesed.
