They cost more. Connect networks that use the same network protocols regardless of Data Link Protocol. They ignore the data link layer info and go ONLY by the network info. Routers forward ONLY the messages that need to go to the remote networks based on the network addresses. If a router gets a packet of data that is not recognized by it as a routable network protocol, it simply ignores it.
Routers are smarter that bridges.
They process information intended ONLY for its address. Bridges just forward ANY info. A bridge would be used if you had a network protocol that was not routable, i.e. you wanted to send ALL the info from 1 cable segment to another cable segment regardless of what TYPE of packet it was, just as long as the mac layer addresses were there. You would use these if you were running multiple protocols on 1 lan that another lan had to see. It is a combo of hardware and software.