Why Can't we add bots to URF custom games?

Server health.

The main reason Riot gave for not releasing the 1v1 map is that the server-side resources involved are pretty much the same in regards to a 1v1 as they are a 5v5 map. At least from what I can remember.

With URF doing a number on the servers, Riot probably doesn't want to encourage some 5 man matches when it costs the same resources as a 10 man match.

More like, why can't URF custom games stay in the first place.

Because of blind draft

Because a product gains value when its on limited time, people will appreciate the mode if its not always available. Also this means that Riot doesn't need to reinvent URF each year, just minor balances and maybe some little additions.