Angawi estimated that over the past 50 years at least 300 historical buildings had been leveled in Mecca and Medina, another Muslim holy city containing the prophet's tomb.
Wahhabism, Saudi Arabia's dominant doctrine which promotes a strict narrow interpretation of Islam, was largely to blame, he said.
“They (Wahhabis) have not allowed preservation of old buildings, especially those related to the prophet. They fear other Muslims will come to see these buildings as blessed and this could lead to polytheism and idolatry.”
http://www.islamicpluralism.org/467/dr-sami-angawi-on-wahhabi-desecration-of-makkah
However, the reason that ISIS want to destroy the Kaaba is that they believe that the end of times, Day of Judgement or Qiyamah is here. Read more here http://en.wikipedia.org/wiki/Islamic_eschatology
It is difficult to understand that they can believe in this, but they do, and they will kill for it.
Read more about the maniac Ibrahim Awwad Ibrahim Ali al-Badri al-Samarrai http://en.wikipedia.org/wiki/Abu_Bakr_al-Baghdadi