Because there is no reason for them to be evenly separated. You're starting with the assumption that they SHOULD be evenly separated, and looking for a reason why they end up being something other than they should be. Your assumption is faulty. And so your question is meaningless.
they arent
The Asteroid Belt
They were never joined to begin with.
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compound
the 4 inner planets are rock planets and the 4 outter planets are gas planets separated by the asteroid belt
an Asteroid field between mars and Jupiter
If all planets are evenly spaced and there are 2,500 planets, they would be about 40 light years apart in the Milky Way.
The are separated by the asteroid belt, between Mars and Jupiter.
all planets arent the same color because they are all made up of different things
Mars and Jupiter.
The asteroid belt separates the orbits of Mars and Jupiter.