The rising popularity of Bitcoin and accompanied by the circulation of more than 1,500 other digital currencies triggered the high crime in the sector that has not yet had the regulation.
So not necessarily Bitcoin specifically, but rather cryptocurrencies as a whole? Bitcoin is the most popular so it wouldn't surprise me if it was targeted more often, but I imagine cyber criminals don't have much of a preference when it comes to stealing stuff. Bitcoins are just more widely available and is an actual household name so there are more opportunities in it for them.
But yeah, I'd even go a step further to say they don't have much of a preference at all for what they steal, it just so happens that there are a lot of opportunities for them within the crypto space at the moment.