Back in the early days of the MCU, it was still figuring itself out. That's one of the reasons that Iron Man 2 is essentially a pointless movie that adds little of value to Tony Stark's story and, aside from the introduction of Scarlett Johannsson's Black Widow, offers little of value to the MCU in general.
The same could be said for its primary villain, Ivan Danko, a.k.a. Whiplash. Yes, he did grab our attention at first but that's only because he was played by the great Mickey Rourke, who was enjoying a huge career renaissance at the time. Trust me, if you see Mickey Rourke stalking you on a racetrack with two mechanical whips, you'll be paying attention too. That said, the movie failed to make the most of Rourke's renewed star power because it failed to make the most of the character.
While Whiplash had a solid backstory, he was essentially just a hitman hired by Justin Hammer to do his bidding and ultimately take down Tony Stark. Sure, it makes sense that two bad guys with a common goal would unite, but if we learned anything from Batman and Robin, it's that this can backfire dramatically. And it did in Iron Man 2, because Whiplash's goals played second-fiddle to Hammer's vendetta and fans haven't really forgotten about how much of a one-note character he became.
Granted, Marvel Studios has gotten significantly better at crafting memorable villains (even if some of them remain flawed), but the fact that Whiplash - never mind Whiplash played by Mickey Rourke - was so forgettable makes this one feel like one of the biggest misfires in MCU history. The stakes were lower in Iron Man 2 so it has been forgiven, but fans have moved on by pretending that it didn't happen (or they have genuinely forgotten that it did).