When the Chochomim decided to appoint R' Elazar his wife felt he would not be suitable since he lacked white hair. A miracle happened and 18 rows of white hair appeared. Effecting a nes is not a simple matter. Was this such an important consideration to warrant a miracle here? After all, a little earlier on 27b when his qualifications were being discussed -- eg. he was wealthy, had zechus avos,etc. -- this point didn't come up. R' Elazar here could have assured his wife that the Chochomim feel he is fit without white hair. They obviously felt he fulfilled their needs in a way the other candidates didn't so a miracle of this scope seemingly wasn't needed.
Yitzchok Gesser, Mexico City, Mexico
It is clear from the Sugya that Raban Gamliel was very stringent with regard to the Kavod of the Nesi'us (see also Rashi in Sanhedrin 12b). The reason he was so stringent was certainly because if there was a lack of Kavod for the Nesi'us, then the Machlokesim would multiply, and the Kavod of Torah would be diminished (like Rashi says in Bava Metzia 33b). Raban Gamliel feared that if someone too young was appointed to be Nasi, the Kavod of the Nesi'us might be completely diminished, and therefore he was stringent.
This is what the wife of Rebbi Elazar ben Azaryah told him -- "they will not look upon you favorably, or with respect, since your youth is evident upon your appearance, and therefore at this stage it is not fitting for you to become the Nasi, lest the Kavod of Torah, Chas v'Shalom, be diminished as a result."
It is interesting to note, however, as you point out, that it took the Binah Yeseirah of the wife of Rebbi Elazar ben Azaryah to point out a concern which the Chachamim did not notice, or did not think was important. Moreover, the Nes which occurred confirmed the validity of her concern.
Kol Tuv,
M. Kornfeld