He didn't say why he left, but I speculate that it is because they cut 50% of his writing for PoE. He only wrote Durance and Grieving Mother and most of his content for them was cut. They were still better than everything else in the game even while unfinished. This isn't the first time he leaves a company.
On the one hand, Chris Avellone deserve to be known as a good game writer. Even though, PoE rather weak in part of companions.
Really don't know, is he responsible for that? No one will know, because it is matter of professional ethic. I think, it is not very good, to discuss the subject.
Any way, everybody has ups and downs.
On the other, He can influence too much on new writer due to huge authority. It will be good, if he brings some sort of challenge for them, like: "Oh, good, I have chance to write really good stuff and kick Chris ass!" And it will be bad, if they will try to blindly follow him.
By the way, Divinity series has... think of words... has its own unique language.
My native language is not english, but I think I know it rather well (for russian) and try to play games in english. For me, to understand text in Divinity: Original sin was harder than in PoE, Wasteland 2, BG and even in Planescape Torment. DOS has more vivid and artistic text. Hope, Larian manage to remain it.
In any case, Chris has already brought to kickstarter couple hundreds of dollars =)
@Hassat Hunter
I disagree, the SP worked quite well, but the game gained a lot of depth, when you played it in MP. Because every new game I started with another player felt different, even when I knew the game in and out. The mechanics are still the same, so I would say Larian isn't weakening the SP, they just put extrawork in the MP, that isn't necessary but appreciated.
I agree, Larian manage to keep SP interesting. I've played only SP - have no friends who like normal rpg =D
Many games have MP or coop functions. RPG stay bihind in that plan, but it will change obviously. The thing is How it will be made?
I hope, Larian will bring live to scheme 4 player coop + Dungeon Master and it will become some sort of industry standart.
Besides it'll bring new game expirience to players, it also can be interesting on streams. Viewer can vout for DM actions =)
By the way, some people could like to be a DM and to provide interesting quests. It can become a sort of hobby for someone. And even more. Now some people can make money for streams, producing game videos and etc. Maybe in future besides of Broadcasters, streamers, videomakers there will be new sort of people - Virtual Dangeon Masters?
@Octu
You forgot
Dragon Commander
.
Yes it is a bit confusing, but they wanted to create big sprawling worlds and games like Ultima

.
But seriously: If you take a look at the history of the company it kind of explains why they did, what they did.
Dragon Commander and
Original Sin were created to found
Divinity 3, until
Original Sin became a big project.
Beyond Divinity was only developed to rescue Larian from shutting down (that's why it's not
Divinity 2), after they got no money from
Divine Divinity. It's quite an interesting lecture if you read the developers diary in the Developers Cut.
When Larian named Dragon commander, The Lord of Chaos was on vacation =)
But they always can make game like "Divinity: Dragon Commander: Genesis: Enhanced Edition: Director's cut" and place it between "Beyond Beyound Divinity" and "Divinity: Original Sin 2 presequel" =D
After that, Swen Vincke will meet Zandalor and they start to gather Council of Seven =)
But seriously: C'est la via =)