Or HaChaim on Genesis 4:4/אור החיים על בראשית ד׳:ד׳

1 וישע ה׳ אל הבל, G'd turned to Abel. The reason the Torah does not first report that G'd did not turn to Cain and his offering, seeing he is reported as having been the first one to bring an offering, is to underline the gulf that existed already between G'd and Cain. Abel's offering caused G'd pleasure and still G'd did not turn to Cain's offering as a result of being in a favourable frame of mind.

2The reason the Torah repeats the word אל twice in the sequence אל הבל ואל מנחתו is, to make it clear that G'd displayed favour to the person bringing the offering and to the offering itself. The reverse was true of Cain and his offering.