Because most of them labor under the impression that the free market solve everything better than government. Under that theory if you have two isp, one selling your browsing habit the other not, if the free market (consumer) decide they want to protect their privacy, then they will flock to the one not selling and the other one will have to adapt or die.
Naturally the clever reader will immediately see the flaws in that. There are captured market which are not free (like isp) therefore it is illusory to think the free market will be better at handling such situation, practically this means once they (any politician) relax the law, it will be used by ISP and you will have no recourse (either you go offline, or you accept the violation of privacy, most folk in the US don't have a choice of ISP).
Rinse and repeat with other free-market-is-better mantra. Like healthcare where it is very obvious the free market system cannot work (every actor except consumer wanting their slice of the pie, captured market as you often not have choice, additional intermediate useless actor like insurer, impossible to negotiate in bulk like a government of reasonable size can etc...).
personally I think the politician are well aware that it will only profit firms and the folk which lobbied them for it (you don't think the idea comes out of good heart or from study of the current situation, right ?). But they do NOT care. They are in for the power and money, not really out of good heart. Yes I am cynic, comes with age and length of observing such behavior (it is not a specialty of the US, it is just more obvious with the GOP).
Unfortunately there is a sizeable amount of folk in the US which still believe in such crap tale, and still vote GOP, (possibly because they can't stand the opposite side, or they can't stand those uppity colored folk or can't stand women having a right to decide to abort etc...). Then there is gerrymandering , while it is used by both side, is far more used by GOP to protect entrenched politician. Finally there is the historical stupidity of the EC and winner-take-all...
So expect more and more of such nice laws as long as senate/president/congress are mostly republican.