Ransomware was here before Bitcoin, but the fact is that after Bitcoin became more popular and thus more valuable, the amount of ransomware exploded and people were seeing the negative effect as they downloaded some bad stuff.
Bitcoin being valuable shouldn't be a reason for the explosion in ransomware. In the end, hackers are interested in extracting the maximum amount of money they can from people. If it were possible previously, they would have just asked for 2X$ of wire transfer, instead of X$.