Why does the Cessna 172S Skyhawk use only two blade fixed pitched prop?
You're not wrong, but a fixed pitch two-blade prop has a pretty big advantage: it's cheap. Cheap to buy, cheap to install, cheap to maintain, cheap to repair. 172s are predominantly trainers in FBO fleets, and cheap is the winner. The efficiencies of a better prop are lost when you're just doing pattern work and short cross countries.

Also, when you're not climbing, a two-bladed prop is actually going to be more efficient, not less as it's creating less drag. According to the linked Hartzell FAQ:


  Depending on the combination of [engine power, operating RPM for the propeller, diameter limitations, design requirements, etc] a 2-blade propeller may be most efficient, but as power increases additional blades are generally required to efficiently utilize the increased power.


This means that on a 160 horsepower engine like the 172 is rocking, you aren't going to see particularly tangible benefits from increasing the number of blades. Going to a constant speed prop would, on the other hand, give you greater utilization of power across the entire flight envelope.

For example...

An overhauled McCauley fixed pitch 2-blade propeller that fits a Cessna 172 is $2,100 on ebay. It has no moving parts (I mean, aside from itself) and is relatively easy to balance.

A (non-overhauled?) Hartzell 3-blade constant speed prop off a Piper PA-46-350 Malibu Mirage is $15,000 on ebay with no logs, which is usually a bad sign. It's not a direct comparison to, say, the prop on a C182, but it's probably in the ballpark. It's harder to balance, and requires more maintenance to make sure the constant speed mechanism is up to snuff.

I was only able to find prices on ebay, which isn't the most scientific comparison ever, but it's a start.