Earlier quoted context omitted.
One reason is because they've found that simply boosting with the original mRNA gives a massive spike in protection even against Delta, apparently on par with variant specific versions of the vaccine. Another is that recent studies are showing that natural infection provides much more comprehensive, long-lasting immunity against all variants than the vaccination does, while vaccination with the current vaccines offer…
I haven't seen any research that says that infection acquired immunity is stronger or longer lasting than from vaccine. The opposite in fact.
There's been some mixed signals coming out of the research, but that's in part due to how immunity is being measured. There are definitely some studies showing that particular antibody levels for vaccines can be higher or longer lasting than natural infection, but recent research is showing that those with natural infection have had a far smaller risk of reinfection than those vaccinated, even when controlling for comorbidities. There are still confounding factors, such as behavioral differences between the groups, but its a pretty stark difference. It also makes sense from a theoretical standpoint, as acquired immunity is based on more than the just spike protein, unlike the vaccines, which is a big part of the differences between variants like Delta. The thought is that natural immunity is therefore more resilient to changes between variants.
Its all early enough that its still an open question, but there's definitely early evidence that natural immunity is more resilient than vaccination only.