I'm by no means a progressive democrat and I am certainly no fan of Obama. In saying that, it's absolutely necessary to understand republicans are pushing for this just as much as democrats. It's not a red vs. blue issue, it's a red and blue issue.

To answer your question, there are probably many reasons, but one reason is because corporations are paying politicians. Here is a story from The Guardian that explains the payoff http://www.theguardian.com/business/2015/may/27/corporations...

Also, here is another story from the Huffington Post that describes the corporate influence on the TPP. http://www.huffingtonpost.com/dave-johnson/now-we-know-why-h...

One last comment, while it is indeed up to our politicians to vote yay or nay on this, it is absolutely up to the people to keep our politicians accountable. Right now, the American people are for more concerned about feelings and witty social media posts to care about the TPP. When Americans stop caring about politicians and stop holding politicians accountable, it's quite easy for them to pull something like the TPP.