Off the top of my head: a possible in-universe explanation is that Luke very rarely went to Mos Eisley, if at all, and probably spent most of his time at the homestead. If that's his frame of reference, it makes sense that he would find Tatooine a dull old rock. Plus, he's a law-abiding do-gooder. He has no interest in the criminal dealings on the planet. Also, the establishing shot of Mos Eisley where Obi-Wan gives a bit of exposition ("you will never find a more wretched hive of scum and villainy" as to the kind of place they're about to enter implies that Luke is unfamiliar with it. That, of course, ignores the fact that the line is there more for our sake as a precursor to the shenanigans we find in the cantina.

But that's putting too much thought into it for my tastes. I simply offered this as a possible explanation.