Why Karaism moved closer to Rabbanism

I recently posted two pieces on the origin of the Karaite sect (I, II).

I came across an interesting review from 1938 by Karaite expert Leon Nemoy of a book on Karaism by Zvi Cahn.

Before I post the relevent excerpt, it should be mentioned that as Jewish, religious insiders, heirs of Rabbinic Judaism we do not realize this, but if put to the Martian test*, many scholars do not think that Karaite and Rabbinite halakhah are actually that far apart.

That the first premise of Dr. Cahn's reasoning, the essential parallelism of Karaite and Rabbanite jusispudence, is correct, no one would attempt to deny. But the important point is that this parallelism is not intentional, but the result of lack of choice, for Anan and his immediate fore- and after-runners seized every possible pretext to differ from Rabbinism,and al-Qirqisani furnishes ample evidence of the ludicrous extremes to which this frenzy for being different had led. As time went on and these wild absurdities were eliminated, Karaism came nearer and nearer to Rabbinism, not because the early schismatics wished it so, but because the later Karaites could not do otherwise without violating clear ordinances in the Holy Writ or simple postulates of common sense.

Review by Leon Nemoy of 'The Rise of the Karaite Sect. A New Light on the Halakah and Origin of the Karaites' by Zvi Cahn, The Jewish Quarterly Review, Vol. 28, No. 4, Apr., 1938

The postulate that Rabbinic Judaism, that Torah she-be'al peh violates too much the of plain sense and intent of the Torah is unfounded. When push comes to shove, to make Judaism work Rabbinic Judaism or something like it has to enter the picture. Not a bad defense of the historical validity of the rabbinic tradtion, if you ask me.

*How would a Martian view it? In other words, what does it look like?