Their simply aren't enough nobles for them to ever compose more than a fraction of the Aes Sedai. Plus, being the most visible and likely to be tested actually works against them by culling the channeling gene from them. That's why obscure corners of the world, like the Two Rivers, ended up producing such a sizable and powerful crop of channelers.
Elayne, Moiraine, Elaida, Kiruna, and Sionid are all nobles and very powerful in terms of AS strength ... Then you have weaker women such as Daigian and several Nobel women mentioned in the series who didn't become AS ... Iselle (wasn't that the name of the Malkieri girl that Merean killed?), Lord Aglemar's sister is mentioned as having the ability by Liandrin and Moiraine. I'm certain there are many more instances of AS being noted as Nobles as well. Seems to me the Nobles are likely all far more inbred and kept the ability to a higher degree than broader groups of commoners.
Come to think of it Logain was a noble and there is the Noble for Tear who is also noted as quite powerful ... So seems to me there may be far more who are nobles than we think! And given how small the numbers in the Tower are it wouldn't shock me if a fair number of AS (pre-series) were Nobles.