Because this means there is a connection problem between you and the servers. The servers are up and running, so I’m guessing it’s something on the route there. You would also be seeing these disconnects in Quick Play, but that queue works differently: when you don’t connect to a match, it just finds you another one, seamlessly. I’d suggest trying the connection troubleshooting steps, and post your test results here, so you can start on a solution for this issue.
This thread explains everything and answers all questions, including why everything else works fine and not Overwatch: Common Technical Issues and Solutions