Well let me tell you why. The only reason this build works is because of the Frostfire passive and Mythic passive especially. Going Frostfire will help you survive and farm the laning phase with ease thanks to the immolate passive. However, what makes the build really work is the Mythic passive. The passive gives you 100 hp and 6% bonus size for every legendary item you have. To give you a better picture, Imagine every crit item you bought had 100 hp slaped onto it like you have an Ornn on your team. While not much, that bonus hp can make a difference, especially when you are building 4 other crit items. The rest of the build is just made to synergize with Nasus as a champion. Essance Reaver for the Sheen passive and Mana sustain. Bloodrazor for the life steal and shield, which also scales with hp. Navori Quickblades for the ability haste to end it all off. The last item can be whatever crit item you fancy. Same with the boots, it all comes down to preference.