Booking agents (especially good ones) get bombarded with emails allllllllll the time. If you don't know how to fill a room, why would they give your email any attention?
However, if you know how to connect with the venue owners, you may be able to open up for a band that does know to fill a room.
Instead of focusing on he booking agents, focus on building a relationship with other acts in your city that have a good size following. Not so large that they'd ignore you, but enough that they're still humble.
It's much easier to get gigs through other bands than through a venue. Just be sure to return the favor by hooking them up when you get a gig too. NOBODY likes a musician who just has their hand out all the time without anything to offer in return.
If you can't bring monetary value to the table, learn how.