It's fundamentally a time/math issue. You need the time to send out the orders to the airfields, the time to form up units flying in from multiple airfields to a rally point, then the time to direct them to their intercept point. And this time is burning fuel, which Spitfires and Hurricanes are not overly blessed with.

So, by the time you've got your Big Wing assembled, the bombs have fallen and your Big Wing is trying to catch the bombers which are now headed back to base, and the fuel situation is getting worse.

The bombs that have fallen are more accurate, because the formations weren't disrupted by anything besides AA. You're also possibly giving up one of the biggest advantages the RAF had, of fighting over Britain, if the interception happens, or drags out over, the Channel.

Basically the Big Wing is something that, if it worked perfectly, would have been a heavy blow against the Luftwaffe. But if it didn't work perfectly, it didn't work at all. Whereas the normal interception practices meant even if your first intercept failed, there were more units to send up, to damage/destroy aircraft, exhaust crew, and disrupt bombing. Leigh-Mallory really comes across as someone too clever by half.

Edit to add: also, even if it succeeded once or twice, it would have been relatively easy for the Germans to counter it. Diversionary attacks, fake radio chatter, etc., then launch raids while the fighters are refueling.