In my Universe, the Colonization efforts and collapse of Earth leads to the rise of organic populations, nations, and cultures all across the Solar-System. Every planet and moon with hydro-static equilibrium and a surface to stand on is covered in independent states and colonies. After its multi-century hiatus, unification, and re-entry into the space age, Earth finds that only it and Mars possess planet-wide government, making them singularly capable of Advancing beyond Type I civilizations.
Due to the sudden reemergence of Earth onto the Helio-Political stage, their radically divergent political and socio-economic ideologies, and Earth's desire for Ultra-lucrative trans-lunar colonies, Earth and Mars have been in conflict for decades. Given their previously mentioned state of unification, both the Earth and Martian states have an entire planet's natural resources and energy at their disposal, Earth (obviously) having more. This is, however, balanced out by the vast array of Martian colonies in the Asteroid Belt, Jovian Trojans, and Jovian Moons. The nations are both in a complete stalemate, both looking for any advantage that might give them the upper hand.
The obvious solution for both Mars and Earth would be to construct a Dyson Swarm, thus ending the stalemate. However, the creation of such a structure would endow its architecture with such insurmountable power as to never be challenged again, thus altering the aesthetic of my work and handicapping my ability to write realistic conflict for the Victor; Yet I can think of no justification for why either wouldn't just opt for this option that doesn't break the suspension of disbelief.
What logical reason (if any should exist) would be so prolific as to prevent either of two rival planetary states from constructing a Dyson Swarm in an inter-planetary setting?
Hopefully useful information ~
The Conflict is one for both Helio-Political and (to a lesser extent) Ideological Dominance
The quality and quantity of Martian and Earth resources roughly balance out (Earth has a far greater human population and energy resources, balanced out by Mars' vast array of mineral resources and Industrial Output)
The first state to control the most of the sun's energy would have resource and strategic superiority, along with greater political leverage, allowing them a greater capacity to win both cold and hot conflicts
Due to lack of W.M.D's extensive enough to wipe out entire planetary populations, M. A. D. does not exist in the conventional sense, but in the sense that if Both powers engaged in prolonged total war, both would inexorably be destroyed
Both Earth and Mars are single state Kardashev Type I civilizations
Earth and Martian colonies are prolific throughout the solar system, with almost no terrestrial bodies lacking Earth and Martian colonies of some sort