When you want to connect a 活用語 (inflectable word) to something else, generally you inflect it to the 連用形 (continuative form). However, with 形容詞 (i-adjectives), there are certain times where you affix things to the 語幹 (stem) as opposed to the 連用形.
〜そう (Affixes to 連用形 for 動詞, but 語幹 for 形容詞.)
〜すぎる (Affixes to 連用形 for 動詞, but 語幹 for 形容詞.)
My guess is that these suffixes want something "noun-like", and since the 連用形 of 形容詞 is adverbial, it doesn't work and instead the stem ended up serving that function.
But, this is just a guess, so I'd like to know if anyone has a historical explanation of some of these things, or more examples to make that argument sound more convincing.