Oznayim la'Torah: Because, after hearing from Bil'am that Mo'av would only fall to Yisrael in the distant future, his anger abated and Bil'am no longer needed to escape. Perhaps one can add that he liked Bil'am's plan - which enabled Mo'av to parcipate in the postponement, thereby repairing their broken relationship.

Targum Yonasan: Because Balak didn't just go home; he set up the daughters of Midyan ¹ in shops (and taverns - Targum Yonasan in Pasuk 2) by the cross-roads from Beis-ha'Yeshimos up to Mount Chermon, in which they sold flax at cut-prices, acting upon the advice of Bil'am ha'Rasha'. ²

Oznayim la'Torah: The Torah writes by Balak "le'Darko" - and not "le'Malchuso" implying that he did not return to his position as king, a. because he failed to stop Bil'am from blessing Yisrael instead of cursing them, and b. because, now that Bil'am had prophesied that Mo'av would only fall into the hands of Yisrael in the time of Mashi'ach, they had no more need of him as king. ³