Here's why:
* Adjectives describe nouns, and they often have degrees of comparison to indicate the level of the quality they describe.
* The three degrees of comparison are:
* Positive: the basic form of the adjective (e.g., strange)
* Comparative: comparing two things (e.g., stranger)
* Superlative: comparing more than two things (e.g., strangest)
However, "strange" is a subjective adjective, meaning it describes a feeling or perception that is highly personal. There is no objective way to measure "strangeness," so it doesn't make sense to compare it in the same way you would compare, for example, "big" or "fast."
You could use phrases like "more unusual," "more peculiar," or "more outlandish" to describe something as being more strange than something else, but "stranger" and "strangest" aren't grammatically correct.