Boat tails???

[Blktail]

I have been looking at the Sierra website for bullets to load in my .308 and 30.06
I am just starting in the reloading hobby. I thought that you always want the best ballistic coefficient that you can get in a bullet, especially if you do any longer range shooting. The boat tail bullets always have the best BC.
Given that, why aren't all bullets made this way? What is the trade off? Do you get the same velocity initially, or does the shape of the base cause reduced initial velocities.

Anybody have a short version on the way to choose a bullet?

Thanks