Because it's a valid idea as to how the universe might actually be explored by thinking beings in a reasonable time frame? It is somewhat misnamed - John von Neumann came up with the concept of universal assemblers but not of using them as space probes. The probe concept might date from as late as the 70s.
http://www.daviddarling.info/encyclopedia/V/vonNeumannprobe.html
The name of the object is 'Oumuamua by the way. If it were really a Von Neumann probe, I would expect it to slow down and harvest raw materials to make copies of itself. Perhaps its inherent protocols might forbid this if it detects that a system appears to be home to intelligent life. Probably it's just a rock with a bizarre aspect ratio that we have yet to explain.
I wasn't discounting the idea but the claim is rather interesting...