The Tur gives the explanation of why grains get a mezonos:
Tur O.C. 208:
וה' מיני דגן שהן חטים ושעורין וכוסמין ושבולת שועל ושיפון שהן גם כן חשובים שנשתבחה בהן ארץ ישראל שכוסמין הן מין חטים ושבולת שועל ושיפון הן מיני שעורים ועוד יש להם מעלה כי עליהם יחיה האדם ואם עשה מהם פת מברך עליהם המוציא הלכך אפילו לא עשה מהן פת אלא תבשיל כגון מעשה קדרה ודייסא ואפי' עירב בהם דבש ושאר מינין מברך עליהם במ"מ
And the 5 species of grain ... also have the significance that the land is praised by them... and furthermore they have another level, since they are the primary sustenance, and if you made bread out of them you would make Hamotzi. Therefore, even if you don't make bread out of them, rather a dish ... you make Borei Minei Mezonos.
What I understand the Tur to be saying (supporting this would take a lot more background) is that mezonos, since you could make bread out of them, they tap into the significance of bread, and when they are altered to an improved state they get an improved blessing as a result of that significance.
This does not exist by the other species. The aspect that they share, which is the significance of the land, is what the Tur says gives them their מעין שלש special after-blessing.