It's not actually all that different since neither Riven nor Blitzcrank appear in each other's splashes.
Therefore, even if they had their own full splash arts, you wouldn't ever be able to realize the difference.
There's no reason to complain about this. At all. None.
I particularly don't play or value either champion but I think people were expecting or wanted a full splash so the characters are given more space to spread out and do all sorts of poses.
Just my take.
I for one really like the splash, but one complaint I have seen that I can understand is the background is kind of bland, and some people feel like it was a missed opportunity to showcase the flavor of each skin.
I like the throwback to classic fighting games, but still I get why some people might want a different style.
