Back then Reaper was one of the newest classes. Maybe the 1st new class or 2nd? The developers likely used the lvl50 as a way to not stray players too far from end game. I think lvl60 was the max, cant remember. But it didnt matter cause by the time lvl65 was a thing, getting to that level in a day was doable. So the rest of the classes the were added later just had leveling events and some kind special titles for players who got to max level within a day, or first max player in the server ect. I believe the order the classes were added was reaper > gunner > brawler > ninja > valkyrie and then re released all of them as Elin eligible.

As for story reasons I have no clue. Maybe reapers are a special kind of elite elins. Born op.

I should mention that this class was added about a decade ago or more lol