The thing is with Rock and Steel is that while the pokemon of both types tend to have high defense and low HP, Rock as a type is geared far more to the offensive than what Steel is. Take a look at Coballion and Terrakion, which are part of the same trio and are distinguished between each other by their Rock/Steel types; Coballion is much better suited to a defensive role thanks to its bulk and ability to Volt Switch while Terrakion is well and truly known for its ability to just lay waste to everything in its path thanks to its brutal Attack and Speed stats and STAB combo.
As it stands, Rock hits a lot of important types hard but it is in turn hit hard back by a lot of important offensive types. Rock also has a wide array of viable moves and abilities; Stone Edge and Rock Slide for use as coverage, Power Gem for the special attackers, Sandstorm to beef up its Special Defense, Stealth Rock to punish threats for entering and Solid Rock for cutting damage taken down. While Rock has few notable representatives; those that are notable, like Terrakion and Tyranitar, tend to be very dangerous on the offensive or can just sit there and tank physical hits like nobody's business, like Rhypherior.
On the other hand, Steel's focus is on monstrously high Defense and low HP and resisting a huge amount of types; resulting in a type where a good number of pokemon of it are very good picks for a team. Outside of its defensive power, Steel has little going for it; few important types hit hard (Fairy being the only important one not covered by many other types), few viable STABs (Flash Cannon and Iron Head mainly, with Meteor Mash on some pokes) though there are a couple of utility moves like King's Shield for stopping physical attackers and Bullet Punch for priority. Steel has many notable representatives, many of which are extremely devastating. Aegislash, Ferrothorn and Heatran are all examples of extremely good Steel types, which utilize their defensive potential in one way or another.
As of now, Fairy overlaps Psychic to a great deal. The difference between Psychic and Fairy is that Psychic as a type is that the former has a lot more personality whereas the latter feels like a more offensive Psychic created solely to balance out the type chart, with a questionable job done at that. Psychic, as a whole, is a support-oriented type with many moves that support the team and some powerful offensive moves to hit back with if needed. Fairy feels like a type with just offensive power, in both pokemon and moves with little other flavor except similar to what Psychic has. Since this is Fairy's first generarion; I expect Gamefreak to improve on the type for Gen VII, similar to how Dark improved between Gens II and III.
