Because "widely" is an adverb, not an adjective like "heavy". Even among adjectives, generally only those of Anglo-Saxon (and not French, etc) can form the superlative with -est. A good rule of thumb is to form the superlative with -est only with adjectives with two syllables or fewer.

Yes, "widely" or "most widely" is an adverb in the sentence you give. And "widely" would never be a superlative. For superlatives, the basic adjectives in your question are heavy wide We use -iest for to form the superlative of words that end in -y. Therefore, the superlative of heavy is heaviest. We use -est for most other words that have one or two syllables. Therefore, the superlative of wide is widest. But for most words that have three or more syllables, we would *usually* say "most + basic adjective": for instance "most beautiful." And there are the irregular superlatives: good - better - BEST bad - worse - WORST far - further - furthest little - less - LEAST but few - fewer - fewest