Riot Butcher's Bridge was in 2015...

Because there's no legendary pirate skins Riot wants to advertise yet

bmb and the bilgewater even in general was the best thing this game has ever had

Because they can't. They already addressed this. Basically it boils down to the fact that the method they used to make Butcher's Bridge would require them to literally remake it, if they wanted to bring it back. Except doing so will take away from other things, so its not going to happen.

I'm not sure, but i'd guess it'd have to have an event to justify it's return and it was said in one of the Ask Riots (i believe) that the Burning Tides was an interregionally intense event for Riot that was on a larger scale than anything ever before.

I know the map could just re-appear but i'd also like an event with it, personally.

I think I remember a previous Rioter stating that they've updated the way the map assets work in the coding since the Butcher's Bridge event and that they can't just plug that "skin" back in because it's not a skin, it's a whole ground up map, and they would have to update the assets and code to make it work within the current landscape of the way the game processes information. Not sure if this was a legit excuse but seeing as how it's never returned there must be some truth to it~

Also I feel like rotating game modes in general have just disappeared (compared to last year). And the modes people actually like are few and far between. We only got ARURF this past weekend 'cause they flubbed up Clash. :/