I'd also be interested in details on this but I assume the gl.enable() API changes fundamental things about the rendering pipeline. It allows enabling things like depth testing and stencil (both involve an extra buffer) and face culling (additional tests after vertex shader). For blending in particular I think it requires the fragment shader to first read the previous value from the frame buffer. Changes this stuff is probably not a trivial operation and requires a lot of communication with the GPU which is slow (just a guess).

If you want to change blending for each draw call you can change the blending function or just return suitable alpha values from the fragment shader.