The reasons 15 has been reached many times while #7 up to now was impregnable are quite simple. The really super long hitters had to lay up on 7 , or drive it up into Hell's Half Acre and hope for a good lie , NOT !
On #15 not only can some one hit it 350 with no impediments but the second shot can be run onto the green.....in firm and fst conditions the math works for a few guys .
We've talked on site about the anomoly that is #7 , and to me it was unbelievable that no one had gotten lucky or caught the right day to hit the green in all these years. Many of us who could never reach 15 have the lenght to reach seven. Before the new tees were built, good caddies were very careful to leave a three wood and or/ 1 iron back in the day, today many long hitters hit rescue from the daily tees , as HHH is in play for them.
There are some real big oaks on #8 tee, I occasionally saw a wayward third shot hit same and bounce towards seven gree, which is perpindicular to the tee. Likewise the bunkers often get quite firm , which might allow a ball to bounce into the front bunker or off a sprinkler head and onto the green . Believe me if you watch enough golf almost anything that can happen does. I've seen tap in birdies on #7 from the halfway house , and even made a few from there lol (now that's crooked)
The real truth why it took so long is in Crumps design, which has stood the test of time that prevented seven from being hit. No one goes for the green in two , because it just doesn't make sense. The risk is far greater than the reward. That's why I thought one of the assistants playing at after work might do it when chasing the last ras of light. When I worked there, we oftern played 1-9 10 and 18 before heading home . I really thought seven would be reached "unofficially " , but kudo's to the hitter. As an aside, it probably will lead to more and more big numbers by players trying to emulate the feat.