Most modern style guidelines say that adverbs, including しばらく, should be written in hiragana. (examples of adverbs which should be written in hiragana.) It is true that some adverbs are simple enough even in kanji, but many people are conscious of this rule and tend to use hiragana versions.
I think this rule came into existence somewhere during the simplification process in writing after WWII. Pre-war documents are full of kanji adverbs. I found that 当用漢字表, predecessor of 常用漢字表, issued in 1946, already said 「代名詞・副詞・接続詞・感動詞・助動詞・助詞は，なるべくかな書きにする」.
According to such guidelines, I believe 石鹸 should be written as 石けん because "鹸" is not in jōyō kanji list. Frankly, I am a native speaker of Japanese and I don't remember how to write 鹸 in kanji by hand. But IMEs are so powerful, and people do not strictly memorize which kanji is in the jōyō kanji list. And such mixture of kanji and hiragana in a single noun is simply not pleasant to the eye. So it's not surprising people use 石鹸 often, and I would probably do the same in casual situations.