I'm speculating here, but early in the book he does mention to Mrs. Weston that Emma says she will never marry. I wonder if that, along with the difference in their ages, makes him reluctant to pursue an engagement. He may feel so certain that he will be turned down that he hesitates to speak at all for fear that it will upset their relationship. After all, since their families are already intermarried, he can hardly avoid her if she rejects him. It could also be that he realizes Mr. Woodhouse really needs Emma there, and he doesn't want to do anything to upset Mr. Woodhouse. I wonder if he would have spoken up sooner had Miss Taylor remained single and living at Hartfield. She would have been a suitable caretaker for Mr. Woodhouse if Emma were to marry and move away.