Because interval adaptions don't progress much past about 6-8 weeks whereas steady state keeps seeing improvements in ability.
You can add interval work on top of the steady state work fairly well, but it doesn't work so much the other way around.
For example, strength confers a lot to endurance but working endurance doesn't confer a lot to strength. When using a particular endurance activity in long duration, you build with endurance volume then add high intensity on top of it.
This is especially true where neuromuscular improvements in running economy occur with extremely high repetitions of movement (hence massive volume of running performed by 5000m+ endurance athletes, etc.), and musculoskeletal CVD adaptations including lactate processing and mitochondria, etc.