For the very reasons that so many people find him so attractive.
Taking account of the fact that I am projecting, I read him as a devotional moralist, two qualities that do not appeal to me. And his "simplicity and humility" feel ostentatious and theatrical, like a king going around asking everyone to call him Harry. It may make him feel better, but it's actually extra work for everyone else. But that, in my two decades experience in the Church, is how pious people roll.
PS. From Fr Z. on humbly making yourself the center of attention...
Back when I resisted the liturgical kissing of my hand when being handed a chain, spoon or chalice (in the ancient Mass), I had made the mistake of imagining myself to be more humble by that resistance. That was a mistake. Ironically, my resistance to those gestures turned the gestures into being about me. Submission to the gestures, on the other hand, erases the priest’s own person and helps him to be what he needs to be in that moment: priest, victim, alter Christus. The trappings, the rubrics, the gestures erase the priest’s poor person. Resisting these things runs the risk of making them all about the priest again.