For the same reason as others have mentioned already.
Over all I like the viper cockpit better and it feels so much more responsive in acceleration.
I absolutely love landing on carriers though so my DCS woykd be incomplete without the 18.
If they release the full fidelity F-15 that will be a must buy in addition to what I own.
I have the A-10 I have barely touched yet but will be fun when I feel like being low and slow.