Hi Third Spearman from the Left,
The CA's explaination is that the non-playable factions had un-interesting tech-tree and did not have a large enough variety of units to make them fun, henceforth they decided to make them non-playable. However, I find this un-true, Macedon had one of the most interesting unit and tech-trees, after Rome and the Selucids, in R:TW yet it was made non-playable. The same is true for Pontos, Thrace and Armenia who have very hard to manage starting positions which, in my opinion, adds to their interest immensly and their playabablility. In my opinion, being able to play as all factions, would have been a wonderful addition to a TW game.

I think the policy of unlockble faction is, however, there to force you to play them first - probably because the other factions are less interesting, this is true for the Romans in Rome due to the senate missions, offices, popular standing, senate standing and the Marian Reforms. If you play as the best faction first then the chances are you will immediately thing, wow, what a cool game and get somebody else to play it, more than you would should you make the effort to play another faction such as The Greek Cities which is resonably dull - phalanx beats all removing the challenge. Although I am against this, I can see the CA's reasoning behind it.

Hope this helps you, cheers!