Why do you think it is challenging to get a serious language partner?
Most of the times the one who speaks the target language better takes the lead and monopolize the practice even if they are willing to practice another language too.
I think the main difficulty is that not everyone takes language learning seriously; some people registering on language exchange sites are more interested in finding a girlfriend online than practising languages. The different time zones may also make the process harder; if a European wants to talk to an American or Australian chat partner, they have to stay up late/get up really early to be able to practice the language. But maybe if it's done once a week, it's not too much of a problem.
If one of the chat partners talks too much, the other one won't have the chance to practise their speaking skills. Listening to native speakers talk in their language is definitely useful, but this should encourage the other person to try and speak themselves. They may be afraid to do so first, because they are worried what the native speaker thinks and they feel stupid when they can't talk fluently right from the beginning, but if they start speaking, first a little, then more and more as time passes, they will gain confidence and be able to talk fluently after a while.
Some people insist on speaking every time, while others aren't always available for speaking, just writing. This may make the other person search for another chat partner. The same for people who don't ask first if the other person is available for speaking and rather initiate a call - and when the other person can't reply, they get offended. But fortunately there are many people who want to practise one or more languages, so everyone can find the people with whom they can help each other the most, while becoming friends in the process.
I would say that many people fancy the idea of doing that but then, when it comes to actually do it, they are not so willing to have patience and listen to the partner to help him.
But sometimes it could also be hard to find someone available to speak on a regular basis and ready to set some time aside for the exchange.
I think there could be many causes that make hard to find someone, aside that the one Matteus commented. One of the most common reasons is the time zone differences. The other person may be not available at the same hour that you, but I think if you plan the sessions in advance, then it won't be a problem.
Other reason is because there are people who are not only looking for a language partner but also an online girlfriend. Among the people I've meet here, there were a lot of them who confessed that they talked with people who was stalking them.
Lastly, because of interests in common. Not all people share the same interests with you, and this may lead to find a more interesting person to speak with.
Maybe I'm too much unfriendly and nobody wants to speak with me again :(