Ramadan is the holy month for Muslims because it's the month when the Prophet Muhammed first began to receive the Quran from the angel Gabriel. In fact, Ramadan was a holy month of fasting BEFORE Islam, and most Arabs observed it. This is the reason Muhammad was outside the Mecca city walls, staying alone in a cave in the first place.
When Islam began to be established, the Muslims continued to observe Ramadan, in the same way the Christians took over the Roman and Pagan winter festival of 25th December.
The reasons for observing Ramadan before Islam are now lost in time.