Today there are malwares that can encrypt data on your hard drive, then demand a money payment within a certain amount of time, in order to get your data back.
With the internet becoming popular, there are also virus programs which hijack your browser, directing you to advertising and business websites.
There are fake anti-virus programs, which themselves contain a virus. They insist that you buy a subscription to get rid of the virus which they put on your computer.
Money has become a prime reason for the criminal-minded to create a virus.
In earlier days, a virus might display a prank message, or cause a computer crash. These were a nuisance, causing loss of work. There were fewer virus programs, and fewer avenues to spread them.