In Phoenix the driving concern (reportedly) was conserving water usage. Not sure about Arlington.
In Arlington I believe it was more a concern about how much light the field would get. And that was somewhat based on the positioning of the field to the sun IIRC.
Edit: Just checked out an article in Dallas News about the surface. Player safety was cited by Ranger brass as a consideration and the author added that the fake stuff allows the club to better use the field for other revenue-producing events.