Part of the paradox is that london is such a big deal partly because, for a lot of the runners, a marathon is a big deal, it is tremendously hard and painful and a lot of work, they aren't naturally good at it and they are (at the same time) trying to make things better for people who are going through such awful times that, comparatively, a 6h mara with blisters would be a walk in the park. That's pretty inspiring and that's one of the reasons why people come out in their droves to cheer the runners on.
Sure there are other marathons, but in them, the proportion of people for who the whole "running" thing is just a foreign country is much smaller. Sure you have slower runners, and first timers, at these, but they will generally be regular runners nonetheless.
And surely it's the support that makes London so special - you can run a lot of the course (OK, on the pavement) any time!