I feel like Tracer in particular was picked for a few, very good reasons, and also that Tracer was picked [i]over a male[/i] also has some good reasons:

1. As you mentioned, Tracer is the flagship character of the franchise. I overheard a conversation in my GAME store a few days back with a kid wanting to buy Overwatch and the mother asking if the girl on the front (Tracer,) was the main character. She's plastered everywhere, and she's the one that most people know of outside the gaming community.

2. People were shipping the [i]fuck[/i] out of Tracer (and the chicks in general.) The chicks in Overwatch are carefully structured waifu bait. Tracer is slim but has a nice ass, Mercy is a downright MILF, Widow is exotic (French accent and weird skin,) Mei is adorable in the Disney sense with her chubby cheeks and large eyes, Zarya appeals to those who like to be bench-pressed by their SO, Sym, Sombra and Pharah all fill the role of Indian, Latin and generic middle-eastern (pretty popular porn genres.) Ana doesn't fit into this list, but it fits everyone else. By making Tracer a confirmed Lesbian, suddenly a whole lot of shippers just got more substance.

3. There are no good male pairings in Overwatch. Wait, hear me out before you bombard me. There is litterally nobody good to have an MxM relationship with in Overwatch. Hanzo and Genji are brothers, Roadhog is designed to be big, bulky and repulsive, Junkrat is dangerously insane, Zenyatta is a fucking [i]robot[/i] who has never shown to be capable of love, Reaper is edgelord supreme and constantly dying/being rebuilt, Soldier [i]could[/i] work, but you have no one else to pair him with. Tobjorn is literally the opposite of sex, Winston is a gorilla... See the issue?

Just my thoughts of why they picked Tracer to be lesbian rather than any other character and why lesbian over gay.