I'm sure it's primarily because caffeine and theanine are culturally accepted and available cheaply over-the-counter. Modafinil is prescription-only (at least in the US), and speed/amphetamines are likewise illegal.

Instead of asking your doctor for psychoactive drugs or delving through the dark corners of the internet or dark corners of downtown alleys, you can get caffeine at Walgreens or Starbucks.