Achenar has been improved. Capitol used to be a Water World, until the most recent upgrade made it an Earthlike. There's still room for improvement, though, especially if/when improved star modelling includes rotational oblation. As far as I am aware, the improved stellar modelling for 2.2 only includes white dwarfs and neutron stars.
Still, Achenar has other helpful features. There are six Empire-aligned factions; becoming Allied with all of them means lots of opportunities for easily stackable missions.
Achenar's main drawback, however, is the sheer size of the system. Capitol and the other Earth-like moons are over 20,000 Ls from the jump-in point. That range deters many casual pilgrims.