Why has al-Qaeda not attacked any Israeli target?

There probably isn't any "why", because the hypothesis of the question is most likely false. The only thing you know for sure is that however many times they tried it, they never got far enough to make the headlines.

Al-Qaeda actually attempted to attack multiple targets in Israel early 2014, but the Israeli Shin Bet announced that they were able to foil the plans prior to their realization. So, al-Qaeda has attempted to attack Israeli targets. It currently happens, though, that both Israeli and Jordanian counter-terrorism measures are relatively effective in preventing successful attacks. Please see the Related Link for more details.