Why was the flow of this sentence changed?

Solution 1:

This word order is a cultural convention (not a requirement of logic). It therefore sounds more "natural" to native speakers.

There is a word order for English adjectives:

Opinion Size Age Shape Colour Material Origin Purpose

The problem: Weight does not fit in here. The principle might be the same, i.e. a convention that says that size comes before weight. I asked this here.

Too complicate things more, there is another rule that exempts the above rule but this happens quite seldom (e.g. it's Big Bad Wolf, not Bad Big Wolf). It requires the order of vowels to be ⟨i⟩, ⟨a⟩, ⟨o⟩ (or in IPA: /ɪ/, /æ/, /ɔ/. For instance:

ding, dang, dong

The first vowel in heavy, /ɛ/, is close to /æ/. So my first thought was that "long and heavy" should comply with this rule. But it does not. This might have several reasons. Firstly, it's not an instance of reduplication. Secondly, the words are separated by and.

Solution 2:

Hard to explain. To a native speaker it just sounds more correct. Long and hard. Long and arduous. We tend to put the long first because long is not necessarily bad, but the second part is. That way the sentence builds to a conclusion.