Mostly because I feel guilty having a $2k slowly rusting grill I seldom use. I feel like I should free up the cash and sell it to someone who will use it more. I burned coal once or twice per month this year, and that will likely decrease as my business travel takes off again. Compare that to cooking 3-5 meals per week on the Genesis or the Blackstone.
Secondary reason is that the Performer is light enough to easily move up and down my sloped lot, so I can cook in the driveway with easy kitchen and outlet access up front, or on the back patio near the lake. I rolled the Summit down to the patio on my own, and I COULD roll it back up to the garage/kitchen level, but it wouldn't be fun.
Also, there are more fun toys for the Performer, like Santa Maria, this pizza rotisserie ring, etc. I love experimenting with tools and techniques.
I love the flexibility and ease of temperature control on the Summit Kamado and it just feels like a premium product. So, I'll mostly likely keep it and just feel guilty about it sitting under a cover 300+ days per year. I've already moved in that direction with the Joetisserie and KJ pizza stone that's too big for any of my other cookers. I just need to fix the ignitor on my Performer and find it a good home.