What is the significance of Commentaries (Atthakatha) to the Suttas? In some traditions commentaries are inherent in the daily sermons and books written by monks. But I've noticed that most of the commentaries are not available digitally in English compared to the Suttas and most people don't use commentaries as reference. 

Are they not so important or what is the view on commentaries?
In Theravada Buddhism, much of the commentarial literature is merely providing definitions and etymologies of words, so it only makes sense in the original pali. Besides this, it is so voluminous that any attempt at translating it would be a daunting task. 

That being said, some of the more accessible and interesting commentaries have indeed been translated into English:


The Dhammapada commentaries (stories only, not verse explanations)
The Jataka commentaries (and the introduction)
The Commentary to Satipatthana Sutta
The Commentary to Sammaditthi Sutta
The Patisambhidamagga and Visuddhimagga
The Therigatha Commentary


I'm sure there are others, but this is a sample.

As to their significance, in Theravada Buddhism, they are what defines the orthodoxy; any interpretation of the canonical texts that deviates significantly from the commentaries is considered to be outside of the fold of Theravada Buddhism. In a word, they are the agreed upon correct interpretation or explanation of the texts.

And they have some funny stories.