I'd put that down to the votes in 1994 having a much different view of catchers than today. Not exactly "sudden" but the product of evolving understanding of the game rather than gut feelings and the player's reputation at the time. Being an exact contemporary of Bench I'm sure didn't help, either, because the comparisons are inescapable.
Not only do I think it is one reason that Simmons got overlooked but I think it is the main reason. Bench is widely regarded as the best catcher in MLB history, with his combination of defensive skills and offensive numbers combined. If I have the numbers correct, there are only 18 catchers in the HOF with Simmons now being the 19th; also a few of those 19 never played in MLB. That tells me that catchers are woefully underrepresented in the HOF. There is no denying the position is physically demanding, more so than any other in baseball. Simmons was a model of consistency at his position and taken for granted IMO. The voters allowing him to drop from the ballot in his first year is inexplicable, unless they were only comparing him to Bench and even then I'm not sure how they reached the conclusion they did.I never saw Simmons play, don’t know much about him, would love to hear more views on this