Maribelle, and Sully come from low nobility. Sully being from a house of knights, and I don't know but I thnk Sumia, Olivia and Robin (well at first) are the only choices for Chrom that aren't of nobility.
The All Father has bequeath me the gift of battle. So now I bequeath you my spear through your heart. The official Eligor of the Shin Megami Tensei IV board
Well in Sully's supports with Vaike she states that she could have laid down and been fed grapes all her life and still been considered a high noble but she feels she needs to prove herself as one (hence her constant training). Basically Sully and Maribelle are the only noble's he can marry.
I am the white void. I am the cold steel. I am the just sword. With blade in hand I shall reap the sins of this world and cleanse it in the fires of destruction