Well for now the main reason is money. Not a whole lot of people have a couple thousand dollars lying around they can use to buy one of each X-wing product. But yeah the product developing strategy means you will have to get ships from all factions to be competitive from the autothrusters in the Star Viper packs to the PTL in the A-wing packs.
Best plan to go if you are starting on a budget pick a primary faction then see what out of faction expansion packs you will need and use that faction as your secondary.
- Build you primary faction to a competitive 100 points standard list
- Get just enough of your secondary faction to make a 100 point standard list
- Don't worry about the other faction too much unless you need some upgrades and it will become your tertiary faction
Soon as you expand your collection you should have a few competitive lists for your primary faction, a competitive list for your secondary faction and enough to make a fun 100 point standard list for your tertiary faction.