Because Teams is not an alternative client to a messaging platform. It is the platform.

You may realize that they are still allowed to bundle Outlook because it is just an alternative client for E-Mail.

Every business which is in the Office365 ecosystem will never use a competitor to teams, because it is just bundled with Office "for free". There were competitors before teams was a thing, but the regulators realized if Microsoft would go forward with this, an entire Market Segment would just die.

Making an Antitrust Case out of this, not only protects current communication system providers but also opens the market up for more competition.