The Pi Zero is single core at 1GHz so it's not a totally fair comparison. There are plenty of times I wish I had more cores, even if they were half the speed. Compiling anything on the zero is incredibly slow. It also has slower DDR2 RAM and no USB3 (which makes a big difference for many peripherals).
But zero is well.... a zero.... If you don't want to mess with arduino/esp*, but want a price point and size in that range, with full linux support, to connect a thingy to another thingy (eg sensor to web/mqtt), and don't care about additional power use, you take a rpi0. I don't think it was ever meant as a "normal computer" or a "server" (as we use normal-sized RPis for).
Sure! The Zero is great as a cheap DAQ, I use them all the time. But the SBC that it's being compared to is much more full featured and I don't think you can say it's worse or even comparable.
