We did apply for a dirt national a couple of years ago but didn't get one at all. We suggested being able to move to the astro in the event of horrendous weather but it wasn't really liked by the committee making the decision I believe, so we went back to astro.
Also there's a lot of time pressure on a two day national to get finished, if we have a period with the covers on and nothings happening it means a late finish or curtailing finals. With a 6 day euros theres room to manoeuvre the schedule for rain breaks (which has happened a lot over many previous euros)(and worlds!).
I just think generally people are happy with astro for nationals, they know where they are with them..
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