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: 
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.
In the Season 3 finale,

> 
>  Peter influenced events in the future, and altered the past. He stated in that episode that, somehow, they *are* the First People - due to a semi-stable time loop, it was the Fringe teams that put the device in the past to be found during the course of the series.
> 
> 
> 

That event linked (or possibly split) the two universes, as the machine exist in both universes. Bolivia only stole what they couldn't find on Over There.

As of mid-season 4,

> 
>  Peter now believes he's in a 3rd/4th universe.
> 
> 
> 

**EDIT:** Events in the series after this answer was posted shows us that the second spoiler is false - he was incorrect.