Gain 10% CDR when you reach level 10. Excess CDR becomes AP or AD, Adaptive.
WhyGrasp of the Undying
Grasp on Malphite allows you to trade effectively with auto attacks in lane. Each point of health is especially important/strong on Malphite thanks to having so much natural Armor.
Bone Plating on Malphite allows you to endure your weak early laning so that you're able to hit your first few items and become unkillable.
If you manage to make it through the early game, Conditioning's bonus resists make a monster and incredibly difficult to kill in the mid game.
Overgrowth's bonus health scales incredibly well with Malphite's natural Armor stacking. Each point of health you earn from Overgrowth is that much stronger thanks to your resists.
WhyThe Ultimate Hat
The primary reason you pick Malphite is for the incredibly strong engage potential with his [R]. The more often your [R] is up the more chances you have for kills and safety when getting ganked.
Malphite's natural build consists of a lot of Cooldown Reduction items and Transcendence allows you to overcome any fear of over-capping CDR. The extra bit of AP you'll get from over-capping is welcomed.