Why on Earth is the new game mode marksmen only? Also no runes?

It's not a bad game mode in design, its actually a really cool idea. But why restrict it to marksmen? I didn't really enjoy blood moon being assassin only (and even then many viable champs were excluded) but marksmen are even more limited. Namely in the way they play and build. It's all very same-y. Especially without the need for anti-tank or defense items.

And of course there's the fact that a lot of people (me included) think RIOT has a large bias favoring marksmen. Current Ez, Twitch and Trist for the last several months, etc.

And the runes? I'm not honestly too upset about that part, but it seems really weird to introduce them and then immediately make a gamemode where they're disabled. Balance issues? Or that everyone would end up with 1 of like 2 pages?

So what gives? I'd really like to try this mode with other champs.