The obvious reasons really. My system is running really nice on W7, it's all set up for reliability and performance with a lot of programs installed. There's nothing to adjust, change or tweak; it's at it's optimum. Also, my workflow is productive with things the way they are. I don't want to risk that by installing W10 on the top of it.
I like to install Windows fresh; I never install these inbred operating system updates, which is what updating W7 to W10 feels like (it was like this with my SP3). Also my main SSD C: drive is getting to be too small at 480GB (2x240GB RAID 0) and I want to change that to a single 1TB SSD.
Thus I'm holding on until I get a 1TB SSD, understand how to fresh install it and have the time and opportunity to be without my main rig. Then there's the additional time required to install all the programs, optimise the system and then get a productive W10 workflow going.
I suppose the reasons aren't because W10 is a bad operating system, it's just not the right time for me at the moment.