I'd like to point out that those verbs also have "regular" forms of 連体形 and 命令形 even though not used quite often. As an extreme example, when making up the expression 連用中止法, they must be conjugated regularly to 連用形.
本日はご来場くださり、誠にありがとうございます。 "Thank you very much for coming today."
but 「本日はご来場ください、誠にありがとうございます」 makes no sense.
Therefore, people tend to think those verbs aren't very different from regular 五段 verbs, and try to account for their irregular forms by phonetic change (both forms) or sometimes by suppletion (only 命令形).
Phonetic change (reduction of R): kudasari > kudasai, kudasare > kudasae > kudasai
Suppletion: in this idea, ください is seen as short for the imperative expression くださいませ, which is a combination of the verb くださる in irregular 連用形 and the auxiliary verb ます in 命令形.