Monopolies destroy the market mechanisms vital for capitalism to work.
Unfortunately for many theories and schools of economics, it turns out that capitalism destroys the market mechanisms supposedly vital for capitalism to work.
The markets - and capitalism - go on working all right, but not along the lines of Adam Smith's fairy-tale "hidden hand". Oh no.
Free markets go either of two ways. Either they remain entirely free and unregulated, in which case they sooner or later evolve into "robber baron" markets dominated by players like Microsoft and IBM. Or else governments step in to regulate them, in a process that soon comes to resemble the Ptolemaic system of astronomy - adjustments to adjustments to adjustments, while the whole thing becomes steadily less stable and credible.
Our current system is a compromise between raw capitalism and socialism. You can argue that it has the strengths of both, or the weaknesses of both, or both. One aspect that has recently hit the headlines is the tendency to privatise profits and nationalise losses, thus giving rich speculators a free run at even greater wealth.
Well, if you were an influential politician, what kind of friends would you have - rich ones or poor ones?