They gave an explanation on Reddit, but I'll quote the full thing here in case you don't want to give traffic:

It just became too much. We spent 2 years watching TI over and over, on repeat. We eventually reached a breaking point, where we had to decide to keep going, and end up resenting the greatest tournament series in esports, or to aim for a clean break with a good ending. We decided to do the second.

But we also felt like we had told the stories we wanted to. Things were starting to get repetitive. I mean, just how often is there a point to me telling you about pushing at TI4 and Wraith Pact at TI11. I understand it would've been cool to get to every hero, and I think we could've done a better job at selecting exactly which heroes we made videos about (I would've liked to get around to Mirana), but ultimately, I'm satisfied with the results.

To me, things ending on their on terms is a blessing. I rather get to choose and be in control, than get swept away by external forces.

Thank you very much for watching the videos. I hope this explains our decision a bit better.