HTTP2 has multiplexing implemented on top of TCP, which is not multiplexed. This means one stream can be delayed due to head-of-line blocking from another stream getting retransmits. Changing TCP to support this is complicated due to transparent proxies that would break, and difficult to upgrade segmenter implementations.

The benefit is not limited to CDNs, any long fat network where we have high throughput and high latency (e.g. most cellular connectivity) should see improvement. I would guess that CDNs may benefit less actually since they tend to terminate near the user and would generally have lower RTT. A typical modern web page has many resources to be fetched, and by making them independent the user experience is likely to be significantly better.

Disclaimer: work at google, not on QUIC but am familiar with the QUIC project and team.