Why is it so difficult to integrate with vBulletin?
 

I'm noticing that there's just not much that will integrate with vBulletin nicely.

Namely, Joomla/Community builder....

Exactly why is it so difficult to integrate vBulletin with other software?

Not sure how you can expect an answer for this...

Because they are different applications, made by different companies, with different code standards, different designs and different ways of achieve the same task?

The only similarity is that they are both written in PHP.

it seems like there are a ton of other integration options out there. People have written bridges for many other forums to work with the various CMS systems. However, it seems that it's much more difficult when it comes to vBulletin. Evidenced by the fact that several individuals have tried to integrate various other software with vBulletin, but haven't been successful.

Even SalesForce doesn't have integration with vBulletin....