Gfyuki

In chapter 43 of Pride and Prejudice by Jane Austen, when Elizabeth visits Pemberley with her aunt Mrs. Gardiner, she looks at the splendour of Pemberley, and thinks:

“And of this place, I might have been mistress! With these rooms I might now have been familiarly acquainted! Instead of viewing them as a stranger, I might have rejoiced in them as my own, and welcomed to them as visitors my uncle and aunt. But no,”—recollecting herself—“that could never be; my uncle and aunt would have been lost to me; I should not have been allowed to invite them.”

Why did she think that? Why wouldn't she be able to invite her aunt and uncle over if she married Darcy and became the mistress of Pemberley?

In his botched marriage proposal at Hunsford Parsonage (chapter 34), Darcy shows his pride and conceit. He believes that his family belongs to a superior social class to the Bennets, and that he would be lowering himself in marrying Elizabeth:

His sense of her inferiority—of its being a degradation—of the family obstacles which had always opposed to inclination, were dwelt on with a warmth which seemed due to the consequence he was wounding, but was very unlikely to recommend his suit.

He expresses this in a manner that Elizabeth rebukes as ungentlemanlike:

“Could you expect me to rejoice in the inferiority of your connections?—to congratulate myself on the hope of relations, whose condition in life is so decidedly beneath my own?”

In his letter (chapter 35) he tries to defend himself but merely provides more evidence for the prosecution:

My objections to the marriage [of Bingley and Miss Bennet] were not merely those which I last night acknowledged to have the utmost force of passion to put aside, in my own case; the want of connection could not be so great an evil to my friend as to me. But there were other causes of repugnance; causes which, though still existing, and existing to an equal degree in both instances, I had myself endeavoured to forget, because they were not immediately before me. These causes must be stated, though briefly. The situation of your mother’s family, though objectionable, was nothing in comparison to that total want of propriety so frequently, so almost uniformly betrayed by herself, by your three younger sisters, and occasionally even by your father.

In light of these communications, Elizabeth believes that Darcy is so proud of his social standing that he would not have allowed her relatives to visit Pemberley, even if she were his wife, for fear of their degrading him.