Wikipedia also says:"this is not a quote by Gauss, but is (a translation of) the end of a sentence from the biography of Eisenstein by Moritz Cantor (1877), one of Gauss's last students and a historian of mathematics, who was summarizing his recollection of a remark made by Gauss about Eisenstein in a conversation many years earlier". Human memory is tricky with context and nuance after many years, especially when one admires a person enough to write their biography, and is focused predominantly on this person.
As for why, Archimedes founded mechanics as a science, and anticipated ideas of integral calculus by two thousand years, Newton revolutionized physics and invented calculus, Eisenstein's work had a much more narrow impact, mostly in algebra. But it was of high quality, and we know from Gauss's own writings that he thought very highly of Eisenstein.
And generally, take everything E.T. Bell writes with a grain of salt.