They would be too CPU intensive. To keep routing updates from being transmitted onto the same port on which they were received. To provide a path between every LAN segment. To help ensure that messages will arrive over all possible paths. Bridges cannot provide congestive feedback information to end nodes.
Bridges offer no form of filtering.
Bridges create more overhead traffic than routers. Prevent routing loops between adjacent routers. Ensure that information about a route is always sent back over the path on which it arrived. Ensure that hop counts are not exceeded. None of the above.