If we imagine a project with only MIT code that want to add GPL licensed code, the only significant additional legal action needed to be compatible is that you now need to give a patent grant that covers the combined work, and in the case of GPLv3 not distribute the combined work in hardware devices that use access control to prevent the installation of modified versions by the owner.
If neither aspects is an issue then the additional work needed to add GPL code to an MIT licensed project is practically zero.
There are however naturally implication for downstream which in combination of additionally third licenses (such as proprietary licenses) can cause real issues, and the relationship between such downstream and the MIT project could as a result lead to reasons why the MIT project feel like they can not include GPL into their code. There might even be contractional reasons involved which mean not only do you now have three different copyright licenses to follow but you also have contract law involved in order to follow the law and have a legal right to distribute the combined work. I can see how the permissive licenses in those kind of situation feel simpler since they do not interact much with the other legal stake holders.
If a project with only MIT code adds GPL licensed code, then they can't distribute the "merged" new set of code under the MIT license.
Previously, someone could use your MIT code in a closed source product and distribute the resulting executables to their customers without including the source. In the future, that someone would have to start following the GPL rules or stop using your not-MIT-anymore project.
If neither aspects is an issue then the additional work needed to add GPL code to an MIT licensed project is practically zero.
There are however naturally implication for downstream which in combination of additionally third licenses (such as proprietary licenses) can cause real issues, and the relationship between such downstream and the MIT project could as a result lead to reasons why the MIT project feel like they can not include GPL into their code. There might even be contractional reasons involved which mean not only do you now have three different copyright licenses to follow but you also have contract law involved in order to follow the law and have a legal right to distribute the combined work. I can see how the permissive licenses in those kind of situation feel simpler since they do not interact much with the other legal stake holders.