Actually, Charlotte is quite poor. The Lucas’ have a large family, and well Sir William can afford to live a life of leisure, he doesn’t have a dowry for his daughters or ample resources for all his sons. His title isn’t heiritary - it will die with him. Regency estates were almost never split up amongst sons, much less daughters. At 26, Charlotte is in danger of being a burden on her brothers (as Jane and her sister Cassandra were while they lived), or having to find a job as a governess (a horrible job that the Bronte sisters did - Agnes Grey is a portrayl of what life was like as a governess). What the humor in Austen hides (which the movie spelled out, and the Bronte’s write about) is people sort of on the edge - particularly women whose only respectible profession is marriage. Austen writes of Charlotte’s engagement:
“Without thinking highly either of men or of matrimony, marriage had always been her object; it was the only honorable provision for well-educated women of small fortune, and however uncertain of giving happiness, must be their pleasantest preservative from want.”