Why are there no other intelligent aliens in the Battlestar Galactica universe?
Co-producer and writer David Weedle says:
Rubber-headed humanoid aliens have always had a place in sci-fi and always will have a place. I'm sure others will find inventive and revealing ways to utilize them. But because of Star Trek and shows of that ilk, they have become a cliche and we have challenged ourselves to create a compelling drama without have to resort to lizard men from deep space.
Also, Edward James Olmos, playing William Adama, was very reluctant to accept sci-fi clichés (including rubber-headed humanoids), so much so he jokingly threatened:
The first four-eyed monster I see, I’m going to faint on camera, and you’re going to write me off the show.
The series is about humans, about being human. Intelligences other than human or cylon would seriously dilute the show's message. Also note that BSG is not the only space opera without alien intelligence: Firefly springs to mind.
Oh and, in your comments you wonder how is it possible to have complex plant life and no animal life whatsoever on New Caprica: yeah, well, personally I always thought they showed the plants on New Caprica and the Algae Planet as way too complex organisms. It's a bit of a bad science in the show (bad science peaked around the Algae Planet arc, the "star cluster" part was painful to watch), due to budget constraints and undereducated set designers.
Frankly, who is to say that there is not? The Lords of Kobol certainly fit the description of "intelligent life." WHile they are worshiped by the Colonials as "gods", from the technology that was shown it is clear that they were likely an extremely advanced race whose morphology was not the same as human beings.
Also, while the series does indicate that space is a vast place, its vastness is still underestimated by the writers. The distance between New Caprica, the Algae Planet, Kobol and the Cylon's empire might only be a few dozen parsecs. Outside of that area, dozens or hundreds of races could exist and simply no nothing of humans or Cylons or be so xenophobia that they have no interest in interacting with them.
Finally, it's clear that the Cylons are obsessed with humanity. They couldn't care less about space exploration or discovering other lifeforms as it does not fit in with their fixation on humanity. As for human beings, the core colonies were their home. WHile they built a series of highly functional societies within that sphere, in the re-imagined series (unlike the original) there seemed to be little interest in travel much beyond that area of space.
I prefer the reimagined series precisely for this reason. In the Original BSG, the very first episode involved sentient Bugs. But instead of going the rubber-forehead route, the New BSG focused on the human element, and told interesting stories about people.