There is no explanation given. The O.N tells us Mr Bennet calls on Mr Bingley as he intended to and IIRC, gives him tickets for the Meryton Assembly. I think the whole idea of allowing Mrs Bennet and the girls to go the dance was presumably so he could have a whole evening in peace in the parlour or library, just as he chose. I'm not completely sure of this, but it appears a married woman could chaperone an unmarried one, so Mrs Bennet could escort her unmarried daughters. In a small town like Meryton where the Bennets are well known and have relations present at the Assembly, I presume this is a satisfactory arrangement. I was thinking it maybe a comment on how Mr Bennet viewed social assemblies- he'd rather stay home with a good book!