PowerPC is not a bad architecture but it has lost the race.
Its very simple...
There are two main companies building POWER/PPC CPUs.
These are IBM and MOTOROLA aka FREESCALE.
IBM is _only_ interested in the high end server market.
Therefore they only produce high end servers.
Freescales PPC customers are telecom/router manufacturer.
So Freescale of focuses on producing chips which are ideally suited for there customers.
Doing a CPU development costs roughly about $50,000,000 USD.
Neither IBM nor Freescale have any customers using the PPC in the desktop / home -pc market segment. So taking a lot of money and building a CPU for no customers - makes really no sense to them.
The Antaris aka 970 aka G5 CPU was a desktop version of the IBM POWER-4 CPU.
Since then no desktop-CPU was build.
But IBM did continue to improve the POWER chips,
After POWER4 came POWER5, then POWER 6, then POWER 7, and now you can buy POWER 8.
So IBM did improve the POWER continuesly. And continues to do this.
But in the "user" segment you do not see this.
The same is with Freescale. Freescale continued to improve their PPC offering -
but in the market segment which their customers did demand.