Around which era?

If you're talking about post-Sassanian Wars during the reign of Heraclius, the reason for a lack of sacking is that both the Byzantines and the Sassinid Empire in the mid 600s (the beginning of the Arabic Muslims based in Medina) were very much weakened with wars against each other meaning that neither the Byzantines or Sassanids could launch any offensive against Medina or Mecca.

By 661 with the rise of the Umayyad Caliphate, (which spanned from Spain, across North Africa, Egypt, Mesopetamia and the Arabian Peninsula) the capital of the Umayyads was moved from Medina to Damascus.

Likewise, after 750, the Abbasid Caliphate moved the capital of Islamic power from Damascus to Baghdad.

Although Medina and Mecca were definitely important cities, the concentration of power in Mecca and Medina, from what I know wasn't altogether there. Not to mention that incursions into the Arabian Peninsula altogether didn't happen. I would posit that none of these aforementioned groups were capable of launching an offensive against the Umayyads, or Abbasids in any significant degree including the Latin Christians or members of the Holy Roman Empire during the Crusades. Not to mention that the Crusaders in the 1200s had issues holding Jerusalem to Edessa. The amount of resources that would've been needed to sack Medina or Mecca would've been enormous and most likely impossible.

As for why Mecca and Medina weren't sacked, they weren't the capitals of either the Umayyads or the Abbasids. Whereas Damascus and Baghdad were actually sacked (a few times if I remember correctly.) Although much more qualified members can fill in information I most likely have missed, simply put, Mecca and Medina weren't the focal points for any of the Caliphates that emerged during the rise of Islam. Not to mention that prior to that there was no power that could actually reach Medina or Mecca given that the Arabs in the 600s were powerful enough to make incursions into Byzantine land.