Question

What makes the city of Mecca so holy that Muslims face in that direction when they pray?

Answer 1

The Qibla (Arabic: قِـبْـلَـة‬‎, "Direction", also transliterated as Qiblah, Qibleh, Kiblah, Kıble or Kibla), is the direction that should be faced when a Muslim prays during Ṣalāṫ (Arabic: صَـلَاة‎). It is fixed as the direction of the Kaaba in the Hejazi city of Mecca.

Answer 2

Mecca is faced during prayer because it is where Kabbah is located. Muslims visit their during their Hajj migration, it is also where the prophet Muhammad was born.

Hope this helps! :)