One reason why some words are written in kana instead of kanji is that it is more convenient for the writer. Writing “ここ” in Japanese is far more simpler than writing the stroke soaked 此処　koko kanji amalgamation.

When first learning Japanese, my first preference was to make everything into kanji too just because I thought it looked a lot cooler that way; but the important thing is to communicate information, and ここ does it faster than 此処.