Pedaling in Rear Wheel design I had built a bike similar to this one before. I think many would have this idea, too.


                
            

However, for some reason, not many of these designed are commercially marketed.

https://sites.google.com/site/pendalbike/pendalbike

There are several advantages on this design:


More aerodynamic posture
Better up-hill climb efficiency (personal experience)
Less maintenance for the drive train
Less Frame materials (weight are transfer directly through the pedal
to rear wheel, and through fork to Front wheel)


There are disadvantages: 


Fewer gears (I doubt this, technology in this Information age should
be more than enough to design a robust Internal Hub gear that support
a pedal axle in the middle)
Cost: Obviously it is not in the stage of mass-production, so the
cost would be high. But I do not see why it should be more expensive
(if not to say cheaper) than a normal bike.


Unless I missed many unforeseen disadvantages, what else could prevent this design from becoming popular in the last 10 years?
What are other disadvantages in comparison with a normal bike that can level the advantages I have stated?
One problem is that the design would not be appropriate for technical riding requiring speed.  Getting behind the seat to push against the pedals would put you out over nothing, instead of out over the back wheel.  The design would be less stable because of that and consequently unsuitable for technical riding. The puts it out for mountain styles of riding.

On the road side, the UCI has been fairly quick to ban anything as non-traditional as this.  While materials improvements and the like are generally the norm, major changes to the geometry of a bike is combed over closely and often thrown out.  

New designs take sometime to take hold (Fat bikes are a good example from recent years).  The fat bike market lingered for years because very few companies were producing the tyres, bottom brackets, cranksets, hubs and rims it needed to grow.  Although most of the other parts were standard road and mountain bike parts, key components can be a hold back.  Once these bikes started getting raced regularly, the market and manufacture of the specialty parts took off.  

For the design you are talking about, I don't see enough appreciable advantages for the commuter market to champion it.  Since it's not appropriate for the other markets, I can't  see such a radially departing design gaining any traction.