Because it comes at the expense of learning another subject that the student may prefer, rather than one that is likely to become a dead language soon.
It doesn't come at the expense of anything, Irish schoolchildren learn all the same subjects that their english counterparts learn, they just have more homework to do that's all. And Irish is not likely to become a dead language soon, with or without compulsory teaching of it.
Regarding the governments motivations, I'm inclined to agree with what you said earlier, particularly as Irish is a minority language now
Its been a minority language for several hundred years. For some of that time it was illegal in Ireland to teach Irish in schools, so where's the harm in correcting that by ensuring that all children are taught a functional level of the language now? Especially when the Irish tax payers, who actually pay for the schools, want it that way.