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.

Sales reps bribe doctors, plain and simple.

https://www.vice.com/en_us/article/pg7ez9/pharma-reps-are-ba...

If there is once choice, doctors prescribe it. If there are two or more very similar choices, doctors prescribe one or the other. Adding competition decreases price, sometimes there may slight differences in side effects. May not be worth of the research itself, but getting into market justifies it.

Example:

Great discovery: PDE5 inhibitor helps with erectile dysfunction. Product: sildenafil (Viagra). After the mechanism was invented, tadalafil (Cialis), and vardenafil (Levitra) were pushed into market and FDA approved. There are roughly different 10 PDE5 inhibitors and the market is saturated.

Sometimes they pay other companies not to make the unpatented drug: https://www.ftc.gov/news-events/media-resources/mergers-comp...

More tricks here: https://hbr.org/2017/04/how-pharma-companies-game-the-system...