It's because for the past two decades Apple has always provided a Power Mac or Mac Pro at the inflation-adjusted $2500-3000 price point. As of now there is no user-serviceable Mac at that price point.
The people who are upset are not necessary those Mac users who have been clamoring for the mystical "xMac" for the past 15 or so years. We know that the chances of Apple ever releasing a sub-$2,000 tower (which is what the "xMac" was) is very slim. No, the people who are upset are people who want user-serviceable, upgradeable, and expandable Macs who were able to buy them at the traditional $2,500-$3,000 price points who now feel abandoned by Apple because Apple's remaining desktop offerings are the Mac Mini, iMac, and iMac Pro, none of which satisfy the expandability requirement of the 2012 and previous Mac Pro nor the user-serviceability requirement of the 2013 Mac Pro.
I have a 2013 Mac Pro and I'm upset with Apple because I don't want to buy a Mac Mini or an iMac yet there is nothing comparable to the Mac Pro at the $3,000 price point. Of course my Mac Pro is still working and I can still get a few more years of solid use from it for my needs (although I was really looking forward to this Mac Pro announcement after years of waiting), but when it's time to upgrade, unless Apple reverses direction (which I think is slim given that Tim Cook has been wildly successful from a financial standpoint), I'll have nothing satisfactory to upgrade to; I'll either have to pick a compromised Mac that is difficult or expensive to upgrade, somehow pony up $6,000 for an upgradeable computer, or leave the Mac entirely. That's why I'm upset, as well as many other Mac users who have waited for years for a Mac Pro upgrade only to be told by Apple that we have been abandoned.