Nobody wants to use N7+ because it's not compatible with N7 designs.
N6 is compatible with N7's designs and has the same rules, so any company looking to do an iteration of an N7 or N7P product will do it on N6. It's easier to go from N7 to N6 than it is to go from N7 to N7+, and you get more out of it.
I believe the main reason is actually there will be a N6 option that allows for: N7+ tailored-IP and N7 tailored-IP to co-exist.
N6 with N7 support risk => Q1 2020
N6 with N7+ support after N5 hits a specific volume target => Any time after Q2 2020
With the N7+ support giving an option for IP on N7/N6 to co-exist with IP built for N7+. Basically, N6 is a long-term funnel node fixing the distribution problem of having a 7nm DUV and 7nm EUV node. That are largely incompatible with each other, but with the N6 fix is now compatible with each other.
Early 2019 => N6 only supports N7.
Late 2019 => N6 is a superset of N7+.
TSMC's word => "TSMC's N6 technology provides customers with additional cost-effective benefits while extending the industry-leading power and performance from the 7nm family"
Only N7/N7+ are listed above that, which I believe is what the 7nm family is referring too.
Modes of support of the 6nm node:
{Drive 7nm into 6nm}
-> N7 to N6 retapeout => same draw
-> N7 to N6 new tapeout(New M0 routing) => new draw
{Drive 7nm+ into 6nm}
-> N7+ to N6 retapeout => same draw
-> N7+ to N6 new tapeout(New M0 routing) => new draw
{Support both 7nm&7nm+ and full 6nm designs}
-> Combined or mixed N7+/N7 tapeouts and/or completely new pure N6 tapeout.