Though for many people wearing a cycling helmet would not change their present riding practices, there's an expectation children / people on small profile bikes would benefit the most from wearing one, (ie safer,) however there are good reasons not to make them mandatory.

As already mentioned it can interfere with constant checking habit for extra safety, as well as cold weather appropriate headgear. Personally I gave up cycling once compulsory helmet laws were introduced in Australia since for sun protection I had an already heavy enough wide brim weather proof hat. It's probably not that common, but I have a weakened neck from a poorly treated bad whiplash (forced over the edge of road on a bicycle with a wooden road marker being right at the spot my head / neck area would land.) Additionally trying a helmet (not all helmets are the same and possibly there are brands out that have addressed certain problems) I found the straps sitting proud interfered with other distant road noises, not an issue in town traffic, but it would certainly drown out vehicles approaching from behind driving on non urban roads, with many drivers not looking for cyclists sharing that road.