Why focus only on software
Patents affect each domain differently, so opposition to software patents doesn't imply a desire to abolish other types of patents. The main reasons for focussing exclusively on software are:
- A broader scope is more work
- A broader scope leads to having more opponents
- Maybe patents are good for some other domains
Social effects as well as economic ones
Whether patents are good or bad in a particular domain depends on different criteria. Some domains, such as writing software, writing books, composing music, performing math etc. are domains where ordinary members of society participate, individually or in groups. (Analogies) To participate in a field which carries patent risks, you need money and lawyers. In the domains mentioned above, adding patents equates to excluding many important contributors to these domains.
Pointing out general failings and costs of the patent system can be useful because it lets me make the argument: "patents are massively inefficient, therefore they should only be applied where we're absolutely sure that they're worthwhile". Some people think the whole patent system should be abolished, but this is a minority opinion among anti-swpat campaigns.
Choosing our opposition
On the other hand, by criticising a wider category of patents, we increase the number of opponents we have to debate. By fighting software patents, we have to fight Microsoft, IBM, and other large companies. If we fight all patents, then we also have to fight the whole pharmaceutical industry and the car manufacturing industry. So we make it harder to achieve our goal.
That said, there's no reason for anti-swpat campaigners to endorse patents in other fields. We don't have to say that patents are ok elsewhere but not for software. We can simply say that we don't have an opinion on other fields, but for software we have a strong opinion that there should be no software patents.