I sometimes saw adverbs instead of adjectives being used to describe things, like for example in estis bele ke vi povis alveni. Why wouldn't one use bela instead? After all, in the sentence you are characterising something ("ke vi povis alveni"): the fact that you could come is nice, so bela.
If I had to guess, I'd say that it is because what is being characterised in this sentence is the verb estis instead, and for verbs you should use adverbs, not adjectives. This is a rule easy to follow for other verbs, for example for sentences like li kantis bele. But, when the verb is esti, like in the first example, what is the rule?