Honestly, plot. The plot required Itachi to be an insurmountable obstacle for Sasuke. However, it also required that the main characters (Naruto, Sasuke) have to work hard and improve at a somewhat limited rate (i.e., Sasuke learns Chidori, then many chapters later gains 3T Sharingan, then hundreds of chapters later gains MS). Itachi was portrayed as an ultimate genius because, as a side character, he needed to appear impossibly strong despite not being the type of character that would be shown training or mastering new techniques.
The result is that, compared to Itachi, the main characters look inept due to making far slower progress... but that's just the nature of Shounen Manga, where power-ups are a critical plot point for main characters and require an arc or so to be invested in obtaining/revealing/testing the new power-up.
