Put simply, Votto's HOF chances hinge on him meeting his career's upside potential and meeting the potential means staying very healthy and productive deep in to his 30s. That upside potential caps at out something like Jeff Bagwell's 80 WAR. An average projection for him comes in closer to 55 or 60.
With Harper, his upside is Frank Robinson's 100+ WAR. Sure, he average projection is probably something like 60 as well, but he's got a lot more room above that than Votto does. (I actually think Robinson is a pretty solid comp for Harper in terms of skill set). Guys who are this good in their early 20's are very rare and tend to have HOF careers. Let's say Harper finishes his age 20 season with 10+career fWAR -- which is what's on pace for. He would be the 20th major leaguer to accomplish that. Of the other 19, two are active (Trout and Andruw Jones), 3 finished their careers in the mid 50s of WAR - Hall of Very Good territory (Vada Pinson, Sherry Magee, Cesar Cedeno) and the other 14 are HOFers, mostly inner circle types.
Look at Votto again, he's on pace for about 33 fWAR through his age 29 season. That's nothing to sneeze at. But there are 159 guys in major league history who have accomplished that. I'm not going to count the HOFers among them, because most of the hitters in the HOF would be on that list. But the list also includes the likes of John Olerud, Robin Ventura, Will Clark, and Nomar Garciaparra -- and a lot of other guys who were really good and didn't quite reach HOF level. As I looked through the list, I was actually surprised at just how "special" it was -- I expected it to be more than 159. But still, what Harper will have done by the end of the season is about 7 times as rare what Votto has done and puts him in more elite company.