there's the possibility of other jedi being DLC and Obi-Wan is badass
see the thing about obi-wan is that he's quiet and unimposing which is also what happens to be his greatest asset
nobody expects him to really do anything but then bam pow he fucks you right up
he beat darth maul, a killing machine who spent his whole life training do murder jedi
he beat general grievous, who on a separate occasion singlehandedly defeated five jedi at once
he beat anakin skywalker, whom pretty much all the jedi said was the chosen one and as such was nigh undefeatable
if it weren't for mace windu, obi-wan kenobi would win the jedi award for 'most bad-ass motherfucker'