Normal isn't much of a defensive type, lacking any resistances and featuring only one immunity. Defensive Normal Pokemon have to bring something more than decent stats to the table in order to be useful over Pokemon that feature actual resistances.
Porygon-Z is not the only member of the Porygon line with an OU niche. Porygon2's well rounded stats and large movepool, including Recover, Thunder Wave, and Toxic, make it a decent supporter. Add in the unique Trace ability, and Porygon2 gains the ability to bounce back Intimidates against Salamence and Gyarados, gain immunity to Heatran and Vaporeon's STAB, and many other cool tricks in the OU metagame.
The game's best special defender, hands down, is Blissey. With a mind boggling 255 base HP and 135 base Special Defense, there is a very short list of special attackers that can 2HKO a healthy Blissey even with maximized Special Attack. Blissey's excellent support moves like Wish and Heal Bell, combined with an array of elemental and status attacks, make Blissey one of the most versatile and useful defenders in OU.
Snorlax may not have the ludicrous stats of Blissey, but its high HP and Special Defense do allow it to take hits from many prominent special attackers. Snorlax can use this bulk to get more opportunities to boost its Attack and Defense stats with Curse, or simply serve as a defensive Choice Band user.