Comment by djha-skin

Quic is mostly used between client and data center, but not between two datacenter computers. TCP is the better choice once inside the datacenter.

Reasons:

Security Updates

Phones run old kernels and new apps. So it makes a lot of sense to put something that needs updated a lot like the network stack into user space, and quic does well here.

Data center computers run older apps on newer kernels, so it makes sense to put the network stack into the kernel where updates and operational tweaks can happen independent of the app release cycle.

Encryption Overhead

The overhead of TLS is not always needed inside a data center, where it is always needed on a phone.

Head of Line Blocking

Super important on a throttled or bad phone connection, not a big deal when all of your datacenter servers have 10G connections to everything else.

In my opinion TCP is a battle hardened technology that just works even when things go bad. That it contains a setting with perhaps a poor default is a small thing in comparison to its good record for stability in most situations. It's also comforting to know I can tweak kernel parameters if I need something special for my particular use case.