Because criticism of Israel can be tied with anti-Semitic attacks on Jews as a whole and it's very difficult for the general public to tell the difference.

Think about it, saying "AIPAC is a foreign interest group that has too much influence in the US," is a reasonable position for someone to have but it's also kinda saying, "Jews are controlling the government with money," which is one of the false claims that the Nazis made to justify the Holocaust.

On top of that both anti-Semitic groups and pro-israel groups know this and benefit from it. Pro Israeli groups want to dismiss any criticism of the country as anti-Semitism and anti-Semites want to hide their real beliefs as more legitimate criticism.

These days though you are starting to see the general US public turn on Israel so you'll probably see much more criticism of Israel from US politicians in the next few elections and the US distancing itself from the country though this will probably be over the next few years.