Because there's no restrictions on how long you can stay at the Blue Lagoon, going there at any other time than the first available time slot means there will people there from prior time slots.
Considering that on most people stay there for 3-4 hours (according to their official site. Personally I stayed 2.5 hours), going there at 11am means the place is crowded with people from 8am, 9am, and 10am
To get the 8am time slot, go to their official site and book it as early as you can. When I went ALL the available slots were completely booked out 5 days in advance, so I recommend booking 1-2 weeks in advance to be safe.
The other added benefit of going there at 8am in the winter means you get to watch the sunrise. It's a really surreal place to be at sunrise. Pics from my trip there: