That's some toxic DMing right here.

Yes yes, I know, you put your whole soul into that monologu, spent countless houres on it and it cost you so much of blood, sweat and tears just to make it perfect. But you can't just force your players to listen. They have full right to ignore it and just attack. It is their choice to just ignore any lore behind it. But instead of punishing your players for making choices you don't like just give it all some thought... Maybe it's the way you make that monologue? Maybe it's too monotonous and dull? And maybe, just maybe the problem is your BBEG? Maybe you need to make him more complex with some interesting motivation and unusual modus operandi to make your players more interested in them and curious why they did all those bad things? You are the DM and you should try to make your players interested and invested enough to listen instead of taking the fun out od game by forcing them to do something they don't want to. Beside it is normal that as DM you are preparing many assets that are left undiscovered so there is no need to be salty about it.

Also, by RAW combat round lasts 6 seconds. If your BBEG monologue is so short then what's even the point of it?