(Conjecture) Other Changlings dislike "solids" and taking humanoid forms. But it is the form Odo is most comfortable with and knows how to communicate with. So, they chose to take the form most similar to that of Odo's.
Another, simpler explanation is that that humanoid form is the easiest one for a shapeshifter to assume, representing a "default" humanoid shape without any specifics. Other Changelings assume that form by default simply because it is the easiest, whereas Odo assumes it because it is the *only* form he is skilled enough to take.
Odo unlike many other changelings has very little skill at shape shifting likely related to his life amongst solids. Not all changelings on DS9 appear in forms similiar to Odo, look at the Martok or the Romulan Tal Shiar agent infiltrators. So Odo is the only one with that limitation.
As far as changelings in general the more complicated the form the more effort and energy the shifter has to exert. A simple humanoid form like Odo's takes less energy allowing them to remain shifted for longer or to have the strength to assume other forms if they so choose. Darkseid253 21:16, July 1, 2011 (UTC)
I think what the question means is this: At some point in DS9, it is said that Odo's humanoid form is imperfect (i.e. in production terms, his makeup that alters his appearance from that of a human) because Odo's shapeshifting skills are not very developed. Why then do so many of the other, more advanced, shapeshifters also take a similarly flawed humanoid form?
One conjecture would be that although the changelings from the homeworld might generally be more adept, Odo's circumstances have required him to focus a lot of his practice on mimicking humans, so Odo and the others end up mimicking humans with equal fidelity. There are problems with this explanation, most especially the fact that the changeling who led the Dominion war effort in the Alpha Quadrant should have been both very advanced and very practiced as a humanoid, and should have been able to make a perfectly good human mimic.
Honestly I think the show was just inconsistent. Happens.
Again there have been other changelings who assumed very accurate and detailed forms this shows that most are quite capable of doing so. However just because they are capable doesn't mean a shape shifter would use the amount energy it takes to do so. In the case of the Female Changeling there is no reason she would need to assume a complex human form to command her Dominion forces. Once again it comes down to the amount of energy shifting into more detailed forms takes coupled with the Changelings contempt for all forms of solid life.
Actually, according to dialogue in "the search" the first answer is correct. The female changling assumed a form similar to Odo's in order to make him more comfortable around them. It isn't stated why all subsequent changling appearances use this form as well, but that may have been a writing concession done to ensure that the viewer always recognises the changlings.Cpthunt 05:44, July 2, 2011 (UTC)