Part of it is because Trials of Mana never left Japan, whereas FF7 came to the West within a year of its release. Part of it is that the entire Mana series has sold significantly less than the original release of Final Fantasy VII alone. Part of it is that FF7 has a legendary status worldwide, whereas the only Mana game with a strong reputation is Secret of Mana, and even that game lives in the shadow of Final Fantasy VI and Chrono Trigger.
Love and tolerate.