Istari var beordrad att förbli, och också fast i, former av män
En beskrivning av Istari ges i Oavslutade Tales . De fick sina "Men-former" innan de lämnade Valinor och stannade därefter i dessa former.
Gandalf och den andra Istari
Emissaries they were from Lords of the West, the Valar, who still took counsel for the governance of Middle-earth, and when the shadow of Sauron began first to stir again took this means of resisting him. For with the consent of Eru they sent members of their own high order, but clad in bodies of as of Men, real and not feigned, but subject to the fears and pains and weariness of earth, able to hunger and thirst and be slain; though because of their noble spirits they did not die, and aged only by the cares and labours of many long years.
"Who would go? For they must be mighty, peers of Sauron, but must forgo might, and clothe themselves in flesh so as to treat on equality and win the trust of Elves and Men. But this would imperil them, dimming their wisdom and knowledge, and confusing them with fears, cares, and weariness coming from the flesh."
Oavslutade Tales
Hur lätt skulle deras uppdrag ha varit om de kunde ha förvandlats till, säg, mäktiga varelser att ta ner Saurons styrkor? De fastnade i sina "män-former" eftersom deras främsta syfte var att förena goda krafter mot Sauron och inte utmana Sauron fysiskt.
[...]; whereas now their emissaries 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
They came therefore in the shape of Men, though they were never young and aged only slowly, and they had many powers of mind and hand
The Lord of the Rings
Så, för att svara på din fråga, omvandlade Gandalf inte att han inte kunde omvandla. Han hade "klädt sig i kött" innan han lämnade Valinor och förblev i kött tills han lyckades med sitt uppdrag.
Sauron
Sauron var å andra sidan en korrupt Maia, som kom själv till Mid-earth (att gå med i Morgoth). Han kunde anta en annan form i andra ålderen eftersom han inte beställdes (av Valar) för att stanna i en form.
Naturligtvis, senare i Numrons fallfall, förlorar han sin fysiska kropp i vraket och sitter därför fast i en fula form som tills han förlorar den här kroppen > igen efter Ringets krig.
1 Målning av Sauron , gjord av JRR Tolkien