What's wrong here? If you can read Thai, read what it says on the sign. Even if you can't read Thai, you should be able to guess.
Motorcycles are not allowed to cross that bridge. The same is true for bicycles and tuk-tuks.
Altogether 39 Bangkok bridges and five tunnels are off-limits to these vehicles. Officials say that it is not safe for them to travel there. When they do, many accidents occur and they make traffic jams worse.