Difference between '-아서/-어서' and '(으)니까'

grammatical morphemes, '-아서' and '-어서', are allomorphs which are conditioned by
phonology, i.e. inflectional affix(ending) '-아서' affixes to stems that ends with 'ㅏ, ㅗ', whereas '-어서' to stems that don't ends with 'ㅏ, ㅗ', or to the each stem of 이다, 아니다(e.g. 이어서, 아니어서), thus they have no difference in grammatical meaning. Also '-으니까' and '-니까' are allomorphs.

Used in the subordinate clause, '-아서/-어서' and '-(으)니까' are nearly the same when they signifies the cause, grounds of the following clause(main clause), but the slight distinction is '-(으)니까' conveys the meaning of premise, which signifies the cause is no wonder or already known.

the declarative sentence:
정원에 꽃이 '가득해서/가득하니까' 나비가 훨훨 날아다닌다. The butterflies flutter around the garden since the garden is covered with flowers.

exclamatory sentence:
꽃이 많으니(까) 향기가 참 좋구나! What a nice fragrance from many flowers!
꽃이 많아서/많이 있어서 보기 좋구나! How pleasant to see many flowers!

interrogative sentence:
꽃이 많이 있으니(까) 기분이 좋죠?
꽃이 많이 있어서/많아서 기분이 좋죠?

But they have syntactical differences in distribution as the following in:

imperative sentence:

'과자가 많으니까/많이 있으니까' 친구들이랑 (네 과자를) 나눠 먹으렴. Since cookies are many, share 'them' with your friends.
'과자가 많아서/많이 있어서' 친구들이랑 나눠 먹으렴.(x)

propositive sentence: 과자가 많으니까 친구들이랑 나눠 먹자. Let's share these cookies since they are many.
과자가 많아서 친구들이랑 나눠 먹자. (x)


On the other hand than the meaning of reason, '-아서/-어서' signifies the clause containing it precedes in time, otherwise signifies a method or means. , e.g. 돌을 들어서 옮겼다. '걸어서 시내까지 나갔다'.

In addition, subordinate linking ending(연결어미) '-으니' is used for  explaining other fact following statement of certain fact.