Good and bad - suppletive adjectives

Solution 1:

As far as I know, irregularity correlates to frequency of use: as a paradigm ( / set of related words) is more commonly used, it will be less regular on average. Suppletion is an (extreme) type of irregularity, and so it is more common with frequently used words.

The most common adjectives and adverbs in all languages I know have more suppletion than their less common counterparts (good v. well). Pronouns and common verbs are very frequent in many languages, and they are also often suppletive to various degrees, or at least very irregular (cf. I v. me and am v. be).

The reason is probably that regular paradigms are easier to remember. If you have a certain word with old forms and new forms, and this word in not very frequent, you are more likely to forget some of the forms, and iron out some irregular features. But if you use the irregular forms every day, you are more likely to "remember" them, they will be more strongly rooted in the linguistic network of your brain, whatever that may be.