GPL vs AGPL

I've noticed the project is under the GPL license, is this intentional over the AGPL license or is there a reason for this?

The GPL license does can not enforce cloud applications, only when the project has been deployed to a user's system must a derivative project be open-source. The AGPL license covers this loophole by stating the code must be accessible by any users that can in any way interact with the code base including website or web services.

More information:
https://www.gnu.org/licenses/why-affero-gpl.en.html

There is no specific reason but big guys like WordPress, Drupal, Joomla etc continue to use GPL. Why don't they change it?

I'm unable to speak on behalf of others however I might be able to assume for WordPress part. (This will absolutely not be the real reason however.)

For WordPress, I could hypothesize the fact they also offer a commercial version/service which they do not want to make open-source and so the AGPL license would have a negative impact on themselves. (Unless they own copyright to all code in the code base via a CLA or something similar then this hypothesis is invalid as they can just dual-license.)

As for Drupal and Joomla I can't justify or assume.

An example of a project that did this unintentionally and changed the licenses as a result is Bitwarden:
https://github.com/bitwarden/server/issues/23

It's not too important that the license changes if you are aware of AGPL and actively want to use GPL, but as far as I understand, any developer is more than welcome to take this project and make a proprietary version of it while being closed-source so long as they remove any self-hosted elements while it's under the GPL license.

The AGPL license would make it so any interaction (including client/server) with it would enforce the copyleft of AGPL.

If this is understood and intentional, then that's great and there's nothing wrong with it at all. For example
you don't mind proprietary services, but just don't want proprietary versions installed on peoples servers via self-host.

I don't think proprietary services would be a problem, developers should be able to earn money through Akaunting in different ways such as apps, custom development etc so GPL is fine.