This is an educated guess, because I did do some research. It was put in there very early on in the law, at a time when there were cattle rustlers and people spreading poison around. Cattle were very important and so they were singled out, and this may very well have been the first offence of any in relation to animal cruelty. That is why, I believe, it had the bigger penalty and was indictable.
On January 31st, 2008. See this statement in context.