It's not entirely obvious, but notice what else the Omniscient Narrator says about Sir William's fortune:

"Sir William Lucas had been formerly in trade in Meryton, where he had made a *tolerable* fortune, and risen to the honour of knighthood by an address to the King, during his mayoralty. The distinction had perhaps *been felt too strongly*. It had given him a *disgust* to his business, and to his residence in a small market town; and, quitting them both, he had removed with his family to an house about a mile from Meryton, denominated from that period Lucas Lodge, *where he could think with pleasure of his own importance*, and, unshackled by business, occupy himself solely in being civil to all the world."

IMO, what the ON is saying is that Sir William let snobbery and a swelled head persuade him to leave his business before he really should have. He probably should have done what Mr. Bingley Sr. did: work all his life and leave it to his children to climb the social ladder. I suspect that the Lucases are in a very similar situation to the Bennets - the parents have enough money to live on reasonably comfortably as long as Sir William lives, but not enough to give their daughters proper dowries or their sons a financial cushion to get them started in life.