Blind, what it's about can't be understood from an American point of view because the USA doesn't have a monarch.
This topic has been debated over and over in the commonwealth countries that are still under the british crown. The issue is that the royal family usually have their own jets and fly everywhere they want to on vacation at the expense of taxpayers. This goes for governor generals too. Not only is this wasting tax money while they sit on riches, but it also wastes fuel and creates pollution for no good reason. Couple with that the fact that the crown doesn't really have much political power anymore and you get a lot of unhappy citizens. So the fact that she took public transit now, thus also supporting the british railroad industry, which is doing quite poorly as of late, is a very big deal.