I am a little confused by your question, as the answers are rather straight-forward:
Why are the acceptance rates for publications in subfields higher than others?
Well, because some subfields have more active researchers, hence the popular conferences get more papers, hence they also have to reject more. Of course you could argue that these conferences should just get proportionally larger, but this is in practice unattractive. Cloud computing, for instance, is kind of a hot field right now. Acceptance rates for all reasonable conferences hover around or way below 20%. Of course we could accept twice as many papers in each conference to have 40%, but then all these conferences would be huge and take two weeks to run, and there would be even more mediocre related work to keep track of. I don't think anybody wants this.
This leads to the other, related, reason: not all papers are made equal. Hot topics tend to attract a lot more complete thrash than other fields. I am in the PC of conferences in software engineering and cloud / services computing. Both fields are reasonably established, but SE is more of a long-standing, existing field while cloud is a hype topic. SE conferences have substantially higher acceptance rates, but I feel the "floor" in terms of quality of accepted solutions is higher than in cloud conferences (that is, the worst paper at an SE conference is miles better than the worst paper at a cloud conference with much lower acceptance rate).
Does this simply mean that those doing systems research are just going to have a harder time publishing than those in robotics?
Not necessarily. See above - one thing I have learned is that the acceptance rate really is a crude measure to judge the difficulty of getting a paper accepted.
are any steps taken to equalise the playing field between subfields?
Not that I know of, and I see this not as a pressing issue that needs addressing.