Mater Eraqus is not coming back because after Terra fought him, he was really weakened. MX then stabbed Eraqus in the back, draining what little life force energy Eraqus had in him. When Sora in KH1 he stabbed himself, he was seen to have full life energy, despite fighting the possessed Riku/Seeker of Darkness. MX and Vanitas willingly let their bodies disappear, but Eraqus did not.
Did you forget Roxas still has the seashell that Xion made when she disappeared? That is kind of like the whole "Their hurting will be mended when we return to end it." message in Re:coded or the Lingering Sentient Terra made. It is not meant to exist but it does as an anomaly to guide Sora to the real people who need help. I bet the seashell will play a big part in Xion's revival.