Why were garden so important for Muslims?

It is not absolutely true. It depends on where they are living and how living. For example, In some ares in Iraq and Syria population density is low and most people are living in homes rather than in apartments and hence enjoying having gardens at their homes. However, in Gaza, population density is very high and no chance for the Palestinians to live in homes. They rather live in small rooms and/or apartments and tents. so, they can;t have gardens. The Israelis who were occupying Gaza enjoyed vast occupied lands relative to their number and constructed a plenty of separated homes with gardens while depriving the Palestinians even from pure water.

Islamic gardens emphasise the relationship between man and god and have been consistent in their symbolism for at least two thousand years. There are fine examples to be found in Muslim countries and those of the Arab conquests of the seventh and eighth centuries. Geometrical in design, they are usually divided into four by canals and surrounded by walls. They create order within to contrast with the disorder without.