Drug salespeople constantly cajole doctors to prescribe one drug over the other using soft bribes like prizes, kickbacks, junkets, etc. Doctors have very limited time to do research and prescribe mostly on the basis of what sales people recommend and what patients ask for (which itself comes from drug advertisements). The "betterness" of a drug has very little to do with what doctors prescribe.

This phenomenon is quite well-documented so I won't cite sources here.