There are many instances where this happens in Japanese. For whatever reason, sometimes the Kanji is not used with a word.
Here's a few examples:
ありがとう (normal) 有り難う (kanji form)
よろしくおねがいします (normal) 宜しくお願いします (kanji form)
こんにちは (normal) 今日は (kanji form)
Off the top of my head there are four reasons why this happens (possibly more).
Kanji is ...