Question: In *Fringe*, there is the "Over Here" universe and the "Over there" universe that the characters interact with.

Why do only these two universes exist? Dr. Bishop mentioned that there were an unlimited number of universes existing at the same time.

I thought it was because it was the other universe that Walter went into that caused all the Fringe incidents, but in Walters experiments, he created a wall that only looked into the other universe, not any of the other universes.

Why are these two universes so important and not all the others?

Answer: 
The universes are now connected because of the first cross over by Walter, so that's why it's possible to cross over between them in the present (although it continues not being easy for "normal people")

I think that Walter first created the window to that universe by chance, and the connection only intensified once he crossed over to save Peter.
Both Nina and Bell indicate in the Second season that the two Universes are moving towards each other ever since Walter first Bridged the worlds. Hence the snow-globe collision metaphor. Since then travel between those Universes would be easier than between "Over Here" and any other Alternate Universe. Also there are no reasons to travel to another Universe other than to cause havoc.