A better seal maintains the pressure in the house better.

Drafts form either from temperature differential and wind outside pushing in.

Radon coming up from the ground is still heavier than air, so it won't mix in very well if it can't displace anything.

It's not a perfect solution, as air movement from circulation will help it mix in, but a good envelope will help a mitigation system out quite a bit.

If too much air can leak in from too many places your exhaust fan won't be able to keep up and maintain a pressure differential across the whole area and fully cycle through the air volume. Instead of the air mass as a whole flowing towards the exhaust you will instead get mixing of inside and outside air and then pulling of a fraction of that. Ultimately failing to properly cycle the contaminated air and also spending a lot of energy on exhausting what was previously clean exterior air.

With it more sealed so you can get a decent static pressure it is like flushing a clean fluid through a pipe and only really needs a little more than the pipe's volume of fluid to clear it, with it poorly sealed it is more like scooping one cup of water out of a contaminated bucket, then pouring a clean cup of water back in and letting it mix before repeating the process, with the hopeful goal of eventually having an uncontaminated bucket of clean water. It would eventually be clean enough sure, but you might have to go through 1000x the fluid volume to get to a clean enough point.

Technically you could just increase your exhaust fan's power until it creates decent static pressure in even a leaky home, but you would also be exhausting a lot of the energy you put into your climate control system.

There's probably a tipping point between the stack effect (hot air rising and pulling on the radon) and drafts bringing in fresh air diluting the radon.