שאמר ראב"ע הרי אני כבן שבעים...ולא זכיתי
R' Eliezer's excitement with this new Deasha would appear to be somewhat excessive?
The Gemara in Horiyot (4a) discussing derasha that the Saducees did not agree with cites as an example:
דאמרי זבה לא הויא אלא ביממי דכתיב (ויקרא טו) כל ימי זובה
Perhaps in this statement is the explanation behind R' Eliezer's peculiar excitement with this derasha. Lacking a tradition that would explain the "Kol" of Krias Shema to be referring to the night, R' Eliezer was left open to a charge of inconsistency - implying that the Pharisees had no real tradition but rather picked their exegesis based on their own whims). Hence the excitement with Ben Zoma's derasha.