HTTP/2 (and HTTP/3) is vastly different in features (added multiplexing, windowing, HPACK etc). All this transforms a largely stateless connection (in the HTTP/1.1 case) to a stateful one. And in order to maintain the stateful connection, you need to store some data (state, configuration etc), thus all these problems.
Also, in HTTP/2, since multiplexing is added, the protection characteristics are different. For example, if the connections were generated by CDN source draws, you may just allow fewer number of connections each with a large pool of multiplex channels, but if the connections were from direct user access, you may then want to allow large number of connections, but each with fewer number of multiplex channels. In HTTP/1, protection is much simpler, since everybody looked almost the same.