Not too many people go to Israel just for Independence day.
During Passover and the Jewish New Year/Yom Kippur 2 week period, flights sell out quickly both in coach and BF and prices shoot up quickly.
That is driven by the many Israelis that live in the US as well as many non-Israeli Jews that visit Israel during that time.
As mentioned above, it may be cheaper to connect in Europe vs. non-stop from US to TLV.