Further in favour of mindbender? Doesn't that answer your own question as to why top logs aren't using solace? (EDIT; your original post asked why more priests weren't using solace!)
By using penance for evang instead of holy fire you no longer gain that 'perk' of solace, and solace simply becomes a 10sec CD mana return with some minor healing competent. Without 2pc you're required to use holy fire if you want 5 evang stacks, so by making the assumption that holy fire will always be cast on CD, taking solace you are also adding in the mana saved by casting 'free' holy fires. Once you remove the assumption that you will be casting holy fire (ie, because you can get evang stacks from penance, which you want to cast for hps purposes) then you're no longer getting the added benefit of free holy fires. You could say you still have a 'free' GCD every 10sec but in all honestly you could just do nothing once every 10sec with mindbender for the same effect.
I did a double check of my chart to make sure a) I wasn't crazy b) nothing had changed since 6.1.
https://www.youtube.com/watch?v=W_PX...ature=youtu.be (currently uploading)
Just for clarity here's my initital post, I know the chart has been copy pasted a few times so I'm not sure the place you found it had the post with further notes on the subject;
http://www.mmo-champion.com/threads/...6#post32456436
I'd feel quite confident at this point in saying mindbender is nearly always the correct option.
I'd make this statement with a couple of assumptions;
A priest capable of executing perfect solace likely has a gear level that will include 2set and enough haste to boost mindbender past 12.5% haste (where the raw mana returns are equal to solace before involving holy fire).
A priest without the gear level for mindbender to overtake solace is probably not capable of using solace often enough to make up for the difference in mana returns, (1 GCD late and it's already less mp5 than mindbender with 0 haste, not including holy fire).
The rest of the priests will probably be the type to find out themselves or by researching forums (such as this thread!), but I'd assume the above reasons are a large factor to why a lot of quick replies will be 'just take mindbender'.