We are given the answer to this question in Chapter Twenty Five of this excellent Romantic novel. Edgar Linton has suffered from a protracted illness which has kept him in bed for a long time. This has been used by his daughter as an opportunity to directly defy his order and to go and visit her cousin in Wuthering Heights, the other home that lies in the middle of the moors, as opposed to Thrushcross Grange, which is more protected from the elements. However, with the arrival of spring, although Edgar Linton appears to get better and at least begins to go on walks in the grounds of Thrushcross Grange, Nelly Dean observes that he really gains little strength. Note what we are told about the visit to his wife's grave:
On her seventeenth birthday, he did not visit the churchyard: it was raining, and I observed, "You'll surely not go out tonight, sir?"
Thus, because of Edgar Linton's health and the illness that he took so long to get better from, he was unable to visit his wife's grave.