I agree, a 2 minute start time is much nicer and before this year was not really possible.
Here are some of the things that we have to take into consideration that I never knew until I started volunteering for the PPA/PPM.
Why not make it start earlier, say 6:30? That is due to the sunrise. A lot of the volunteers start their day at the summit and hike down. The sun rises at 6:15am on the 3rd weekend of August. It is light up there by 5:50 or so. We don't want volunteers hiking down the peak from the top in the dark. You also have issues with staging volunteers early in the morning at the NoName and Bob's road aid stations. Thus the original 7:00 start time was kept to preserve the original timelines from an operational perspective.
As for the number and spacing of the waves, there was a cap on the time of the last wave too. There are two things at play here: Thunderstorms and the Cog's first trip of the day.
This year was an oddball for weather, but in a normal year we can expect to see a storm arriving a bit after 2pm. With a 7:30 start and the 6:30 cutoff, folks are arriving at the summit as late as 2. Just in time to get out of there before the regularly scheduled afternoon storm.
The other concern was that the Cog railroad had an 8am train, and if we kept ruxton closed for too long, then we would have a huge impact on their operations. Given how much they have supported the race for the past few years (how do you think all of the equipment and volunteers get to Barr Camp each year? ), we wanted to be as nice as possible to a company that has supported us so well.
This year, the Cog is closed for repairs, so the timing of their first train of the day was no longer an issue.
The final thing was that the math worked out nicely with the 20 or so waves. We have about 2100 registered runners (about 1800 who show up on race day). By simply using the first numbers of the bib as the wave number and staging them at 1 minute intervals, everything lines up nicely. Wave 3 has bib 300 -> 399 and leaves at 7:03. Easy to remember, and on race day I like easy. :-)
--john