I think the reason for the difference in 'It was fun' and 'It was enjoyable' is that 'fun' is a noun and 'enjoyable' is an adjective. You can't say 'such enjoyable' but you can say 'such enjoyment'. The adverb 'so' can modify an adjective, but not a noun. 'Such' is needed to modify a noun. Therefore:
So beautiful, so funny, so tragic .
Such beauty, such fun, such tragedy.
The difficulty lies with the evolution of the language (as ever!
), so that these days 'fun' is being used as an adjective - "It was such a fun day" - but we haven't yet completely caught up with this in our heads, and won't allow an adverb to qualify it, even when it's being used as an adjective.