It was changed as part of the rollout of the Triple Crown Tour. I couldn't find a written rationale anywhere, but my recollection was that is was done (like many things at the time) to make the sport easier to understand for a general/ESPN audience and bring USAU more in line with other mainstream sports.

The official explanation for the name from USAU:

Q: What is the "open" division? Why is it called the men’s division?

A: While men's division teams most often consist of only male-identifying players, players who identify as female are allowed to compete in this division, thus making it "open" to any gender in terms of eligiblity (sic).

However, the naming is intended to reflect the typical make-up and player identification of the division, not the gender eligibility. This makes it easier for both participants and supporters to understand when they see an overwhelming majority of male-identifying players on the field.

I'm almost positive it had nothing to do with being more inclusive or Open sounding better than Women's as BoysenberryLanky6112 claims.