How much bandwidth we can use in bridge mode?

Potentially, all of it.

If you want a more specific answer, pose a more specific question.

Why we can't use all bandwidth in bridge mode?

Most likely because you haven't got a pure bridge and are forcing traffic down through the router's CPU. Except on MikroTik's highest-end products, this will tend to bottleneck the process. A big part of the art to configuring a RouterOS box is minimizing the percentage of packets that this occurs on.

Once again, if you want specific advice, pose a specific question. The most helpful thing is to post your /export output, suitably sanitized for public consumption.