Yosef had near-absolute power as second-in-command to Pharoah. For example, after the famine begins, he purchased all the land and cattle in the country in return for feeding the people, and then turned all the farmers into sharecroppers when they asked for more food the next year. There is no hint that Pharoah initiated these policies. It's not even clear that Pharoah was asked to approve them.
Yet in a few cases, we see Yosef taking a much more submissive attitude. When his family comes to Egypt, he has them ask Pharoah to permit them to stay in the land. And in our parsha, when Yaakov dies, Yosef must beg nicely to Pharoah before being allowed to travel to Hevron for the burial. In fact he does not even dare ask Pharoah directly, but rather asks Pharoah's servants who convey the request to their king. Yosef has so much power in everything else that he does, so why is he so timid in these situations?
I think by looking at how positions like Yosef's came about, we can understand the inherent limitations of the position that forced him to be submissive in the above situations.
The basic issue confronting Pharoah was that running a country is hard work, and it's more fun to have somebody else doing it. The dreams and their interpretation by Yosef were simply a good opportunity to do what he probably wanted to do all along. Pharoah was not the only ancient king to make this calculation: for example, Achashverosh gave Haman and Mordechai positions similar to Yosef's.
At the same time, if these chosen subordinates had enough power to run the state, then they likely had enough power to overthrow and replace the king. Thus, the subordinate's loyalty to the king had to be unquestionable. In practice, this meant that while the subordinates had near-absolute control of policy, they were also absolutely prohibited from doing anything to help themselves personally.
So when Yosef wanted to undertake far-reaching economic reforms, for Pharoah's benefit but whose details Pharaoh did not want to be bothered with, nobody questioned what he did. But when he wanted to settle his family in Egypt, the question of his motivations arose and Pharoah had to review the decision personally. And when he wanted to visit his ancestral country - thus perhaps demonstrating his continued devotion to them rather than Egypt - Yosef's motives were all the more suspect, and he had to approach Pharoah indirectly, and to mention the oath to Yaakov which made the foreign burial morally unavoidable and obligatory.