Why does the Island's powers appear as electromagnetism?

I believe that the science explanation is just one way of looking at the island. You can form theories of how stuff works on the island based on the electromagnetic properties and see if it fits. The scientists in the series do just that and offer facts for the viewers that adhere to such a story line and want to analyze it scientifically.

You could also look at the island in other ways, like religion, magic,... Each of these ways of looking at the island gives you another perspective. You can focus on John Locke/MIB to give you some clues to how the island really works philosophically.

However, none of the different perspectives fully explain everything on the island as a lot of things are still mysterious. I wouldn't say that the series failed to explain itself as the scientific explanation wasn't present/complete. That would mean that the writers wanted to provide a full scientific explanation. However, I believe that they didn't want to provide a clear scientific explanation. They just didn't provide any full explanation, as (in life) there're always things that remain mysterious.

So, as for your question, I believe the writers wanted to provide as many perspectives on the island as possible (as everyone has a different view on life). The scientific approach might suit you the best but that might not be the case for a preacher. I wouldn't say "they wanted to be more realistic" as the scientific explanation is not complete and thus leaves scientific viewers wanting more.

As for why they use electromagnetism instead of for example quantum mechanics: I think it more closely relates to how scientific work/experiments are set up. It's easy to measure it and is still one of the fundamental forces in physics. They could have included some string theory and worked with that but given the time setting of the series, it's more likely to measure electromagnetism first and build a theory with that.