My memory is a bit hazy, but I think in the history of viruses, some of the pioneers were avid Apple users who were angry at PCs getting to be more popular. So they started writting viruses as a revenge against their hated PC-using enemies. Then they soon moved to writing and distributing user-friendly programs that let more PC bashers easilly create variations of their viruses to more easilly infect PCs. So right from the beginning PCs had more viruses.
Now things have gotten to the point mentioned above - if you want to do damage, you'll have a much better chance damaging the company with the larger market share.