Because longer rides…in the 3 - 6 hr range build slow twitch fibers and really pack them with mitochondria. Of course It takes MUCH longer to build “power” this way because slow twitch fibers don’t contract as fast as fast twitch fibers. Because these long, slow twitch rides serve to really pack the fibers full of mitochondria it actually improves Vo2 max by an increased ability to use oxygen and an increased ability to recycle lactate before it reaches the bloodstream…meaning that the cycling takes place strictly within the muscle itself, providing much higher fatigue resistance than fast twitch fibers…which are really tapped by Z4-Z5 workouts.
Vo2 workouts “should” really only serve as a peaking tool around targeted races or as top end maintenance (reduced sessions)…since a base of fitness based heavily on Vo2 is a fragile one…because you will have less fatigue resistance in longer races…which means that your FTP won’t matter for very long if it is based almost exclusively on Vo2 work, which really primes the glyocolytic engine BUT with the detrimental effects of increasing the amount of lactate that is produced. Do this too much and your FTP will fall due to the inability of a poor aerobic base to cycle this increased lactate production (VlaMax). This often happens to cyclists who race too much and thus slowly erode the aerobic base they started with at the beginning of the season. Once this happens in earnest, their performances can stagnate or become poor as time goes on.
You are really trying to find a balance between fat metabolism and glycolytic metabolism…a balance suited to the type of racing you like to do. That said, personally I would err on the side of a larger aerobic base than a heavy diet of Vo2 work but that will also depend on how much time you have to devote to cycling.
Finally…a substantial aerobic base will make it MUCH easier to absorb the rigors of Vo2 and upper Z3/Z4 workouts…as long as you make sure and maintain that base throughout the season.