yitayningwut- Its a shvacha raya. 1. Techias Hameisim is considered a miracle, just one that they had the ability to perform. 2. Since its a miracle it is a big deal. Hashem may not allow them to do it again.
Derech HaMelech- I dont think its quite the way you phrased it. Its not like they all had to pass a test to get into the Mishna. “Field trip to the cemetery. Whoever raise the dead gets in the book, those who cant… you lose!”
My understanding is everyone that is mentioned (I heard in the Gemara) is on the Madreiga that they could have been mechaye meisim if necessary. Its just pointing out the Gadlus of those Tanaim.