Dosing. You can order crystalline caffeine, but you have to take micrograms. If you overdose you could have a heart attack.

Availability, coffee is a social and calmly accepted drink wherever you go. Even if you can’t get coffee you can get tea. Finding pills is more troublesome.

Because liquid is consumed slowly because it’s hot, it’s easier to dose to the level of excitement you want. If you ingest a pill there’s a delay effect, so it’s easy to take too many pills before the impact hits you. Sipping hot coffee, you can get to the point where you’re just the right level of jittery and then stop