Why is XOR preferred over XNOR?
$\operatorname{XOR}$ is similar to addition when working on Booleans; by contrast, $\operatorname{XNOR}$ is not a group law if we want to use $0$ as neutral.
Economy for extension: we can build a 3-inputs $\operatorname{XOR}$ from two 2-inputs ones; we can't for $\operatorname{XNOR}$.
Economy for making one: we can build $\operatorname{XOR}$ with four identical 2-input $\operatorname{NAND}$, but that can't be done for $\operatorname{XNOR}$. The construction goes:


$z\gets(a\;\operatorname{NAND}\;b)$;
$x\gets (a\;\operatorname{NAND}\;z)$;
$y\gets(b\;\operatorname{NAND}\;z)$;
$c\gets (x\;\operatorname{NAND}\;y)=(a\;\operatorname{XOR}\;b)$.



Update: the later argument works only because/when $\operatorname{NAND}$ is cheaper, faster, or/and less power-hungry than $\operatorname{NOR}$. That indeed was the case in TTL logic: that source shows 4 NPN transistors for $\operatorname{NAND}$, including one with two emitters, versus 6 for $\operatorname{NOR}$. The same holds for RTL logic made from PNP transistors, which used to be popular when these where made using germanium. However, RTL logic using NPN transistors (typically, on a silicon Integrated Circuit), which favors $\operatorname{NOR}$, has also existed. Famously, the Appolo Guidance Computer was built from 3-inputs $\operatorname{NOR}$ gates, two per IC.