Why did Mr Bingley decline the Bennets' invitation to dinner?

He says that he has business in town.

From the text:

"An invitation to dinner was soon afterwards dispatched; and already had Mrs. Bennet planned the courses that were to do credit to her housekeeping, when an answer arrived which deferred it all. Mr. Bingley was obliged to be in town the following day, and consequently unable to accept the honour of their invitation, &c. Mrs. Bennet was quite disconcerted."