The reason for this is the fact that meditation is simply a technique of focusing the mind so as to open one's self up to spiritual growth. It is not something that would be in any way specific to one religion.
A person can meditate with any number of goals in mind. A Christian might meditate in order to become more open to receiving the will of God. A Buddhist might meditate in order to escape the desires of the body and come closer to banishing all wants from their mind and body. These are very different goals, but they can both be achieved through meditation.
So meditation is not something that has to be unique to any particular religion. People of any religion (or none) can use it because it is simply a vehicle for opening the mind to whatever "truth" an individual wants to achieve.