Why do pine stands exist?
Forest Succession 101



Pines are actually fast-growing, shade-intolerant, early-successional species. When land is allowed to lay fallow after being cleared by fire, wind storms, human land use or other major disturbance, the successional sequence will begin.

At first only herbs and grasses grow, but eventually these are shaded out by shrubs and tree seedlings. In this setting, the limiting factor is typically light. Because pines are fast growing in the sun, they usually take over a field and develop a relatively dense even-aged stand. 



Because the seedlings grow so well and densely, they usually shade out and prevent most other trees and shade-intolerant plants from growing. The result: a dense stand of mostly pines:



I'll note here that early on in this process (before processes of self-thinning really get to take off), you can end up with what are sometimes called "dog hair" stands. Also, if you see clear rows in the pine stand, that's a good indication the pines were planted. 

Eventually (80-120 years), most of these pines will die and fall out of the canopy (both due to "natural" causes and additional wind, ice, pest, or fire disturbance). As large pines fall and clear large enough canopy gaps, shade-tolerant sub-canopy hardwoods can take their place. Because pines are highly shade intolerant, pines do not regenerate again in that stand (without another major canopy-clearing disturbance). The result: a mostly hardwood-dominated forest (with perhaps a few remnant older pines):



So...


The all-pine stands you're seeing are early on in the successional sequence.
The few pines (that "don't look too happy" because of age) mixed with hardwood ("seasonal variety") trees are in a mid-to-late successional stand.