Is there a strong reason that the qbittorrent images are shipped with libtorrent 2.0, as opposed to libtorrent 1.2?

The major change in 2.0 is that it offloaded the disk caching to the OS, but it is plagued with issues.
The memory issues seems to be curbed in recent updates, but it has inherent speed problems, as mentioned by the qbittorrent dev here:

But it seems that the speed problem cannot be solved without providing I/O caching at the application level (which is available in libtorrent-1.2).

• Add "Simple pread/pwrite" disk IO type qbittorrent/qBittorrent#21300 (comment)

Not elaborated there, but the speed problem is that it cannot saturate a gigabit connection.

There is a rewrite of caching in the works for a possible libtorrent 2.1 release, but the progress seems to be slow:

• pread-disk-io arvidn/libtorrent#7013

So with this perf issue I don't see compelling reasons to use libtorrent 2.0, aside from "latest being the greatest".

To prove my point even the official qbittorrent docker image uses 1.2:
https://github.com/qbittorrent/docker-qbittorrent-nox/blob/main/Dockerfile#L30

While LISO and Hotio both gives the option to use 1.2 or 2.0 via release tags or env variables.

So if you don't want to support multiple releases, I would strongly consider sticking to libtorrent 1.2 until 2.1 is out and proven.