The distance means that we cannot send a probe, since it would take thousands of years to arrive. We cannot get useful pictures without telescopes that have mirrors thousands of miles in diameter.
You could say that out technology is not strong enough, but also the the problem is enormous.
-
Alex say: Several reasons.
First of all, an intelligent species may not have developed as far along as we have. We were considered to be an intelligent species even before we discovered how to wireless transmit information (radio).
So with that, there may be an intelligent species pretty close to us...but we'd have no way of knowing it since they've yet to develop any sort of technology that would send a signal. Or maybe they just managed it recently but the signals haven't gotten to us yet.
Secondly, they may be too advanced for us to detect. We're looking for what we know which is radio waves, microwaves or any of the current technology we're using. They may not be using that anymore and are using something that we don't understand.
To use fiction for a moment, In the Star Trek universe, they achieve Faster Than Light (FTL) travel as well as communications by the use of something called SubSpace. Let's say that it's a real thing and some intelligent species is using that. We haven't discovered it, we don't even know it exists. So there could be a ship out there just this side of the asteroid belt chatting away to the homeworld...and since we don't know about their communications...we'd never know it. Not until we learn about it ourselves.
Thirdly...we might be at the same level of development but are so far away that our signals have not had the time to travel the distance from here to there. We've only been putzing with radio waves for around 130 years. If for the sake of argument someone else started at the same time but both our worlds were 200 light years apart...it'll be another 70 years before we learn about it.
-