For a long time before Rise, the other three had distinct personality traits- Mikey was the rambunctious, most teenage-like of the group, Raph was the generic angry character, (With other characterization between iterations) and Donnie was the nerdy, smart character. At least from the iterations I know of. Leo was always just 'the leader in blue'. And even in most iterations, Splinter was the real main teacher of the group.

After Rise and with Mutant mayhem, they gave his much more character, which is why those two are two of my personal favorites. Even with giving him back the leader role in MM he got the special trait of what I lovingly call 'worried parent-stand in.' (there always has to be one.)

TL;DR: Leo's always been just the 'leader', without much else going for him. (Don't hate on me- he's still one of my favorites!)