There are several problems with using HTTP for resource-constrained devices. HTTP is verbose and requires a lot of bytes for headers. These headers are in plain text, and since HTTP has grown over time, there are a lot of headers that need to be supported to achieve compliance with the standards. This forces implementations to become large, which might be a problem if the device has limited memory. CoAP is much simpler and has less options, and therefore has a smaller code footprint than HTTP.
CoAP is an Internet Engineering Task Force (IETF) standard and is defined in RFC 7252: https://tools.ietf.org/html/rfc7252.
At the same time, the amount of data in the payload is often small. A sensor value can be encoded in just a few bytes. The great difference between number of bytes sent and number of content bytes sent implies a great waste. This waste of bandwidth...
