Why is HTTP considered an insecure protocol?

Answer 1

Well, it is not encrypted, it sends plain text over the internet, anyone in the path can read all the packets and view the text sent. HTTPS sends encrypted packets from the server to the client and can't be read by anyone except the sender/receiver.
Internet traffic is sent through many middle connectors in transit and at any one of them your data might be captured.
That is the short answer.