Katakana is mainly used in imported words and onomatopoetic words. The word "マップ" is a imported word so it is written katakana.
I think the basic difference in usage is 地図 is mainly used in Japan and マップ is mainly used for foreign people because 地図 is Japanese and マップ is the reading and writing in Japanese for the English word "map".
In addition, we eager to use English words in many situations like even Japanese news. I think many Japanese may think using English words is cool. When we use the word "マップ”、this may be a main reason.