Because instead of expanding on the ideas that Rhyhorn and Rhydon had going, it introduced unrelated concepts way out of left field. Not only that, it thew away everything that was cool about Rhydon, and some of the choices don't even make sense in the context of an evolutionary line.
For example, Rhydon gained ears upon evolution. Rhyperior took those away.
Rhydon, while bulky, has muscular legs that support it. Rhyperior has stubs that barely keep it off the ground.
Rhydon has a chest plate, giving its body some definition. Rhyperior took that definition and turned it into, quite literally, a ball of rock.
Seriously. Rhyperior's body it a ball. Not rounded or kind of round - it is a sphere.
The cannon arm idea is kind of cool but comes out of nowhere, and they way they're designed seemingly renders Rhyperior's arms useless for anything else. (The Ankylosaurus tail also seemingly somes out of nowhere.) The drill is now angled forward, which just gives Rhyperior a comical Pinocchio look. And speaking of comical, THICK ORANGE EYEBROWS. For the love of Arceus, who thought that was a good design idea.
And then to top it all off we get the nonsensical orange armor bits. Now, if these had been placed on Rhyperior as if to expand the armor concepts of Rhyhorn and Rhydon's plate armor, then that could have been great. But no. They're little chunks of who-knows-what that have been thrown onto a design that is already too fat and bulky. In fact, their placement looks closest to a belt and suspenders, further adding to the comical effect.
So far I've described a fat Pokemon with useless arms and a Pinocchio nose that wears bright orange suspenders. How exactly am I supposed to take this Pokemon seriously?
You realize that not all pokemon evolutions resemble their previous forms, don't you? Your whole post can be described as just nitpicking.
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