I'd have to guess that this is because of ease of use. C++ lets you get as close to the metal as you choose to, so there is no reason why a C++ solution shouldn't be at least as fast as one written in any other language, and yet ...
Of course it also depends on what additional libaries you are using, especially when it comes to parallel/GPU programming in C++, but easy to believe that Julia out of the box makes it easy to write high performance parallel software.
> C++ lets you get as close to the metal as you choose to
This only ends up being true (for any language, but it's too often cited for C++) in a pretty useless Turing Tarpit sort of sense.
So it's not "no reason" it's just sometimes impractical to solve some problems as well in C++ as in a language that was better suited.
Now people do do impractical things sometimes. It's not very practical to swim across the English channel, but people do it. It's not very practical to climb Mt Everest, but loads of people do that for some reason. Going to the moon wasn't practical but the Americans decided to do it anyway. But the reason even the Americans stopped going for a long time is that actually "that was too hard and I don't want to" is in fact a reason.
Yes, with unlimited development time I would expect C++ solutions to be as fast or faster. But Julia hits a really nice combination of development speed and performance that I haven't found in other languages, at least for number crunching and data pipelines.
Of course it also depends on what additional libaries you are using, especially when it comes to parallel/GPU programming in C++, but easy to believe that Julia out of the box makes it easy to write high performance parallel software.