The pacemaker is probably only designed to last for 50 or so years. I'm not an expert, looking up the statistics, that seeems to be a very generous estimation. So you want to put a power source that can last hundreds of thousands of years in a device that's going to last 50, at great expense?
Well, there aren't many reliable chemical battery designs that can last for a few decades. We have stuff that lasts for at most one decade or two, and we have the nuclear stuff.
Of course, if you are using the nuclear stuff, you will prefer something shorter lived, so you can use a smaller battery. But I don't see why that huge objection on using the longer lived stuff too.
