Because the TPP doesn't prevent countries from giving similar favored trade to China.
If Vietnam wants to sign a "free trade" pact with China, it can. Which means the TPP can't actually isolate China.
The TPP offers protections for union organizing, wages, environmental impact, discrimination and for copyright & patentsc etc. The hope is that workers in these other countries will now unionize and bring their own wages up (improving their lives) as well as making the US more competitive
So did CAFTA. We've never attempted to enforce those provisions, and people attempting to organize unions in CAFTA countries are routinely killed. Including during Obama's terms.
Why would the TPP be different?
Also, the intellectual property provisions actually make the situation worse for most TPP countries. They are having to make their patents and copyrights longer, greatly increasing drug prices among other things.
It also has reduced tariffs in other countries. I know Japan had to give up tariffs it had on US auto imports (they're being phased out over five years, I think).
Average Japanese tariff on US goods is 1%. Fluctuations between the Yen and the Dollar are
much larger than 1%.
86% of the GDP covered by the TPP is already covered by free trade agreements. 12% of the remaining GDP is Japan, with those "crippling" 1% tariffs. That leaves an utterly insignificant size of a market to "open" to trade.
In return, the TPP makes it much easier and safer to export
capital from the US. So that US money can more easily create jobs in other countries, instead of creating jobs in the US.
If you're not in the 1%, there is nothing good in the TPP.