Greece was considered a crossroads of many cultures because of its geographical location. Most importantly, Greece was on the Mediterranean and near to Asia.
By being on the Mediterranean, Greece had access to the cultures that ringed this sea. They could have contact with cultures as far to the west as what is now Spain. They could have contact with cultures in what is now Lebanon and Israel.
By being near to Asia, Greece had contact with those cultures as well. Greek colonies in Asia Minor, for example, were very much in contact with (and sometimes ruled by) the Persian Empire. In these ways, Greece was a place in which people, goods, and ideas from many cultures could mingle.