Because the only information Blizz has released on this so far seems to point this direction. If TBC were getting an era, then they likely would have announced this already because they know it’s been highly requested.
The fact that they haven’t announced an Era means it’s most likely that they’re just planning to repeat how the ‘progression realm’ timeline went the first time around. The fact that the ‘classic’ part of anniversary realms are unrolling in phases means that TBC will come in phases also, ultimately ending in a transition to WOTLK.
At the absolute most charitable interpretation, they haven’t decided yet and are waiting it out for later.
Here is the statement that these assumptions are drawn from:
It’s possible if not probable that Blizz legitimately somehow believe that the playerbase only wants to play temporary seasonal/phases, instead of perma-eras for each xpac.
We can really just look at how they handled the classic timeline the first time around, as a sign of what to expect.