Because its an awful comment and the stereotype encourages anger, aggression and hatred towards Jewish people.
Its also not true. The CEOs of Fox news, MSNBC, CNN for instance are all not Jewish. And even if they were, you can make comments about the bias on these news sites without referencing religion or ethnicity.
AIPAC does indeed wield strong political influence, as do powerful people in general. There are also many other lobbies which wield power.
Why is the news coverage so pro Israel? For lots of reasons. The evangelical christian right. The impression that it is the only democracy in the region and the strongest US ally. Historical Western guilt about treatment of the Jews. The tactics used by the Palestinians at times. The general American attitude towards Muslims/Middle Eastern people considering the last few decades of their foreign policy. And yes, the Jewish vote will also play a role, just as it does for other demographics too in some places (ie your views on Cuba if you're running in Florida).
Its not all some grand Jewish conspiracy and comments like those are not at all helpful to the situation, nor do they really help normal Jewish people around the world see it as anything other than an attack on them.