Why the angel has to leave before dawn in Genesis 32:26?

Sarna has interesting suggestions here (in Understanding Genesis):

1. He adds: Centuries later the prophet Hosea refers to this incident and describes Jacob as having striven with “divine beings,” “an angel” (Hos. 12:4-5). ’’The man,” thus has become a “divine being,” an elohim—a generic term in biblical Hebrew for supernatural beings.82 Jacob’s assailant was an angel in the guise of man. He is never, significantly, identified with YHWH. (note 82 is just: 8 2. Cf. p s s . 8:6; 97:7. )

Hundley has pretty valuable insights and objections to the proposal that Jacob’s adversary here is a river spirit or a “demon”, he notes that:

134 Perhaps the man’s desire to leave before daybreak signals not a loss of power (for which there is no compelling evidence), but a desire to keep his identity hidden (Wenham 1994:296).
[…]
While the man is likely divine, his specific identity remains obscure.146
The man’s refusal to offer a name may hint that he is not Yahweh himself, since Yahweh freely offers his name during most encounters, while subordinate divine beings are neither given names nor do they self-identify until very late in the Hebrew Bible (compare Judg. 13:18 with Dan. 8:16; 10:13; 1 Chr 21:1).
[…]
This conflict with Yahweh or his envoy certainly raises some theological issues. Why make the deity so weak, and why flee before daybreak?
Once again, the divine form seems appropriate to the context in which it appears. With innumerable forms at his disposal, Yahweh chooses a weak human(like) avatar whom Jacob may potentially beat. Nonetheless, rather than offering firm answers, the text obscures the nature of the
encounter. Jacob and the reader know that he has encountered Yahweh or a deity so closely aligned that he may be called by his name.
Nonetheless, so much about the encounter is abnormal that, while the reader may identify the man as Yahweh, the man does not encapsulate the deity either in form or in power. Thus, flexibility and opacity together allow Jacob to have a physical encounter with a man-shaped god, without limiting Yahweh to manlike form or powers.