Because it produces an image with a perfectly long and linear curve. It makes you question your life choices. Digital cameras are better at being T-Max than T-Max is. I am half-joking, but it's just too perfect. I once did a photoshoot in the studio with T-Max 400 and a digital body. Then I scanned the negatives using the same digital body and showed the resulting mix to the model. She couldn't tell them apart and neither could I after a few weeks. T-Max films are pointless in the digital era. They aren't "filmy", they are "digital".
HP5, on the other hand, you can't confuse an HP5 scan/print for a digital one.
I am inclined to agree with this,
but - I also find T-max films to be
reasonably malleable in terms of the look they deliver.
Where a film like Tri-X or HP5+ have a characteristic look “baked in” to them, T-max is far more neutral. This is because Tri-X has an S-shaped curve, whereas T-Max 400 has a much more straight line curve. (forgive me if I am overstating the obvious)
Often, photographers prefer a film that has a definite “look” bias; they know their images are going to have a certain visual character if they choose something like Tri-X, and they know how to work with those characteristics. For many, T-Max can appear sterile and overly clean, lacking the qualities that define it as having "film" traits. This perception of “digital-looking” is valid, as T-Max’s lack of distinct traits may not resonate with all photographers.
But I have learned to be thoughtful about which film I use for the circumstances. Sometimes Tri-X (or FP4+) is better suited to how I see a scene, and other times I want that T-Max neutrality for an image.
Some examples, photographing the same scene on different days (but similar lighting conditions):
This photo was made with Tri-X
This photo was made with FP4+
and
this photo was made with T-Max 400
The differences are subtle, but discernible to the trained eye.