I'm not sure you can draw the conclusion that since there is one intelligent (as defined above) species, out of many on earth, that intelligence is unlikely to develop.
The main issue I see is that there were other species that were close to as intelligent us, but we out competed them and they went extinct.
I wouldn't be surprised if there was a sort of "first one wins" effect where the first species to reach a certain level of intelligence quickly (compared to the time it takes to evolve that level of intelligence) comes to dominate the planet wide eco-system on a level far beyond every other species, thus preventing the development of other intelligent species.
Even if we outcompeted the Neanderthals (or other very intelligent species), they were still very close to us in the tree of life (they were cousins). For a trait to be a high probability outcome of evolution it needs to happen independently in many different species - take binocular sight for instance which happens in all kinds of very far related species.
Even if our close relatives had similar intelligence and we outcompeted them, it's still safe to say that intelligence (as defined) appeared in only a small branch of the evolutionary tree.
Why don't we see species underwater building radio stations? There's lots of life diversity underwater and they certainly weren't in our "competing" ground (being underwater) so they could've developed intelligence independently... but didn't. That's my point.
One species having binocular sight does not significantly effect the evolutionary advantage of binocular sight in other species.
And define what you mean by "high probability", I would agree that intelligence (as defined as building radio communications) does not have a high probability of evolving compared to binocular sight.
What I am getting it, is I would not be surprised if intelligence (building radio communications) was like a race. A race only has one winner. Just because a race may have a thousand contestants, does not mean that the probability of there being a winner if 1/1000. Rather the probability of there being a winner goes to 1 as the duration of the race increases.
Or its like concluding that because life only appeared once on this planet, its very unlikely that life could have appeared at all. Another hypothesis is that once life appears, it makes it very difficult for new life to appear by preventing the conditions that lead to abiogenesis. For example maybe chemical evolution can not occur, because in all places where it would the are living organisms that interrupt the process because those chemicals are useful to the organism.
Similarly, I Humans have an unprecedented degree of control over the evolutionary trajectories other organisms. Maybe other apes, if left alone for hundreds of millions of years would evolve into a species that develops radio communications. But organsim's need evolutionary pressure to evolve, and humans really, really like preserving species. If a species of apes start dying off due to some evolutionary pressure, rather than let nature take its course and let the apes which can survive that pressure out compete the others, humans will intervene to keep them around, and as a result they will remain stagnant evolutionary.
And, I don't think it's even correct that we are the only intelligent species on Earth. Most intelligent, yes. But, certainly not the only intelligent.
The main issue I see is that there were other species that were close to as intelligent us, but we out competed them and they went extinct.
I wouldn't be surprised if there was a sort of "first one wins" effect where the first species to reach a certain level of intelligence quickly (compared to the time it takes to evolve that level of intelligence) comes to dominate the planet wide eco-system on a level far beyond every other species, thus preventing the development of other intelligent species.