Just because they didn't become strong enough to reach this level or received extra power boosts allowing them to skip to Mega like Agumon and Gabumon.
But it happens in almost every Digimon season. The main Digidestined characters (like Tai and Matt, Davis and Ken, Takuya and Koji) are more important than the secondary Digidestined characters (all others from the group). They have a more important plot to develop and their partners (or themselves) will evolve more.
- Season 1 - Agumon and Gabumon (WarGreymon and MetalGarurumon, later fused into Omegamon)
- Season 2 - Veemon and Wormmon (Imperialdramon, later skipping into Fighter and Paladin Modes)
- Season 3 - Guilmon and Beelzemon (Dukemon Crimson Mode and Beelzemon Blast Mode)
- Season 4 - Takuya and Koji (Aldamon and Beowulfmon, later evolving into KaizerGreymon and MagnaGarurumon and fusing into Susanoomon)
However, during Season 5 this taboo was finally broken and all 4 human characters evolved their partners into their respective Mega levels and all of them also acquired their respective mode changes. The Season 5 was the only exception.