I believe it's 6 months because up until then the gut it "leaky"- it allows bigger molecules through. For many reasons, to maximise uptake of milk, to allow immune system components through…
Wean before the gut has matured means molecules from weaning foods may not be broken down and absorbed properly, and may lead to some proteins crossing the gut whole- the risk then is triggering reactions like coeliac disease.
Of course guts mature at different rates, some may be ready before. But unless you start biopsying before weaning, the only way to know for sure is to wait until 6 months, when research has shown the benefits of weaning outweigh the risks of the gut being super-slow to mature.
DH was weaned extremely early- if mil is to be believed before he left hospital, so within days of birth (rice/rusks in bottles, jars of baby food). He has awful, awful problems with his guts. Friend of mine started BLW at about 3.5 months "oh I'm not weaning, he's just having a taste"- kid now has gut issues and food allergies.
Of course you'll never know if it is related or not. But for me I'd feel terrible if I weaned early against advice, and then had gut problems, all for the sake of waiting a few weeks.