Because coastal waters mean that in the super-early game, you tend to run into 1 barb ship or none, the Liburna is just *better* than the trireme as well as cheaper to build.
If I'm especially hungry to begin sea exploration ASAP, I'll build a Trireme, or if I want to start accumulating XP to have a veteran ranged fleet, I might build more, but in the early game it's not "do I build a Liburna or Trireme" it's "am I building any sea units at all."