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.
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.