Maiar in physical form

[Earendilyon]

[The Istari] were forbidden to reveal themselves in forms of majesty, or to seek to rule the wills of Men and Elves by open display of power, but coming in shapes weak and humble were bidden to advise and persuade Men and Elves to good, and to seek to unite in love and understanding all those whom Sauron, should he come again, would endeavour to dominate and corrupt. - Unfinished Tales, Part IV, ii: The Istari.

So, that Valar did not allow the Istari to reveal themselves in all their majesty.

As for Sauron being weaker while in spirit form, he had put much of his power into the One Ring, so his spirit was much less powerful as before. Also, he was killed at the end of the Second Age, which means that it was much harder for him to rebuilt his body than when he had relinquished his body out of himself.
Furthermore, incarnated Maiar (Valar also btw) tend to get to love their bodies so much (when they wear it long that they are much more connected to it and even depending on it and can't do without it (see also Morgoth). So, the first thing Sauron would do when loosing his body is to try and rebuilt his body, which takes a lot of (spiritual) energy. During the Third Age it took even longer, because he had lost his Ring.