Because in it is considered poor grammar not to have subject-verb agreement.
From the text of the Pentateuch and Torah as a whole, it is clear that all occurrences of Elohim (plural) and Yahweh (singular) are discussing the same deity. In verses like Deuteronomy 6:4, Literally, this would translate as "Hear and Obey O Israel, the LORD your Gods are one!" The rules of English and context would dictate that you change the translation to "lord your God is one".
Furthermore, the Old Testament is not just a book for Christians, but also a book for Jews. Since Jews do not believe in the Trinity (largely because of Deuteronomy 6:4) it also makes more sense to translate Elohim singular in all occurrences because God is one. Whichever convention is adopted in situations like Deuteronomy 6:4 and Genesis 1, the convention is always used in all occurrences throughout the Torah for the sake of consistency by translators and textual critics throughout the entire translation process (with the exception of where elohim refers to a group of actual lords, kings, or pagan gods.)
Furthermore, It appears in 20:13 that the diety(s) which are the subject in this verse are Yahweh, not Abimilech's gods based on verse 11. In verse 11, there is no fear of "elohim" on the part of Abimilech and Abraham posits Abimilech may kill his wife because of his deception. Clearly Abimilech would fear his own "elohim," so this can only be Abraham's "elohim" who's power is unknown to Abimilech. Since 20:13 is part of the same speech by Abraham and there was no subject change or mention of any pagan or heathen "elohim", it is clear that the "elohim" in v 13 is the same "elohim" as in v 11 (God.)
Therefore, it might be appropriate to translate this as "Gods", but never "gods". Abraham probably recognizes the existence of Abimilech's gods and that these gods have some power, but they are not the God that Abraham follows or is discussing here and are not as powerful Yahweh.