Mostly, because it targets, so it's a lot easier to dodge:
In Masked Heroes, if you Sakuretsu Armor an attacking Anki, they can dodge it by Mask Changing the target. With Widespread Ruin, it doesn't care whether the target is still there. It just blows up the largest monster.
In Blue Eyes, something like Dragon Spirit of White can dodge it by replacing itself with another Blue Eyes whereas WR will probably still kill something.
If they have the resources, Buster Blader can Destruction Swordsman Fusion the targeted attacker into a Buster Blader fusion whereas WR still kills the new fusion.
In Sea-Stealth Attack, SA can be dodged by banishing the target (Widespread Ruin isn't much better in that situation, but at least it can still destroy something).
If you're up specifically against Citadel Whale, it can fizzle SA because it targets, but not WR.
Hazies can't be targeted by SA, but can still be destroyed by WR.
Because the target is fixed from the moment you activate WR, players can also still try to eke value out of the targeted creature before it dies. They could, for example, try to E-Con-take with it, or use it as fuel for a Treacherous Trap Hole. In Geminis, they can Spark the target away, getting a lot of value.