My question is: How did Chazal attain its authority? More specifically, how did the Pharisees who evolved into Chazal beat out the Sadducees who would later become practically extinct (some say that the Karaite Jews evolved from the Saducees because they share similar thoughts regarding the Oral Torah)? The Sadducees were highly opposed to the Oral Torah which at the time ( second century BCE to 70CE) was still not written down. How did the Pharisees eventually win in this matter (by win i mean attain the strongest following and continue to live on as a Jewish sect) while Sadduccess died out? Also, How did the Halachot and discussions of Chazal in the Gamara become the governing laws of most Jews whereas the Karaite traditions and interpretations of the Torah became almost extinct?
Simply - this is the sign that the Pharisees were the ones with the correct tradition of the Torah. HaShem promised in our precious Torah that he would maintain the covenant he made with the forefathers with those that would listen to his laws, observing them and performing them. The Pharisees and their adherents did so, so they were preserved. The others deserted or abrogated the law of HaShem, so they no longer exist identifying themselves as Jews.
In other words, that's the covenant - those that would devote themselves to HaShem's true teachings are guaranteed preservation by HaShem. The very act of adherence to the true message of HaShem is the effort of preservation. That is the system HaShem designed. We do ours, an He promises He will do His.
Where does the Torah make this statement? For one, at the beginning of Parashas Eikev (Deut 7:12):
And it shall be, on account that you will listen to/accept these laws and you will observe them and perform them, HaShem your G-d will preserve for you the covenant and the kindliness which He swore to your forefathers.
It's difficult to know, since there are few historical sources on the topic. However, it's possible that the answer lies to some extent on the dependence of the Sadducees on the temple, and their popularity primarily among the upper classes. This would mean that after the Roman destruction of the temple, those involved with the temple as well as the formerly rich would lose much of their influence.
While the Sadducees are able to persuade none but the rich, and have not the populace obsequious to them, but the Pharisees have the multitude on their side.
But this doctrine is received but by a few, yet by those still of the greatest dignity. But they are able to do almost nothing of themselves; for when they become magistrates, as they are unwillingly and by force sometimes obliged to be, they addict themselves to the notions of the Pharisees, because the multitude would not otherwise bear them.