The official reason is only what we read on the official announcement:
we have also evaluated the role of tutors and decided to remove this position since the opening of bug reports for everyone no longer requires a group with special rights in this regard
Before this change, reporting bugs was basically the only feature that was exclusive to tutors. Other tasks such as reporting statements and posts and joining test servers, which used to be exclusive to tutors, were made accessible to all players long before this change.
If CipSoft kept tutors after this change, there wouldn't be much more to the tutorship other than the status, but effectively speaking being a tutor wouldn't add anything to a player's ability to help others, after all, anyone can reply questions in the Help channel and Support Boards.
As for the Senator position, it's just an obvious consequence. The Senator position has always been used to indicate retired Gamemasters and Tutors, but this was done at the discretion of CipSoft's Customer Support, as a way to make sure only players who actively contributed to the community had this position. With the removal of tutors, they would have to either make all of them Senators (effectively making it meaningless) or go through all of them one by one to decide, which would definitely be a lot of work and lead to lots of complaints.
Outside of the official reasons, there's also the reality of the tutor system, which is that it was pretty much dead. There were no more than 5 tutors actively helping players in the support boards for the past 5 years. In game, most Help Channels are dead, and most "active" tutors were only cheating the system (to become a tutor or to get more TYs and stars). The vast majority of accounts with tutor status were either inactive players who haven't played in many years or, as I mentioned, players who cheated the system but didn't really contribute to anything.
Those of us who paid attention would often see the evidences of the above in the game. Tutors that were asking questions they should be answering instead (e.g. questions that can be easily answered by looking through the official documentation, a very basic thing every tutor should know), tutors that didn't even understand their position, didn't know how to report the most basic of bugs or didn't even know the private Tutor boards existed.
For these reasons, it was very clear for a few years already that CipSoft was leaning towards ending the tutor system. If I had to guess, I would say that it only took this long because they were studying how they would deal with the bug reports after that. For example, it's likely that bug reports will be processed by the Customer Support now, instead of directly by Testers, which was a decision they had to make to go through with this change.