Why don't textile workers in China, India, Bangladesh and other cheap labor countries stand up for themselves and demand to be decently paid for their work?

It has to do with the number of people available to do those jobs in countries like India, China, Bangladesh and likes. If 100 workers in a factory start a strike and threaten to quit or stop work (which is the only way the management will even listen to them), there will be 1000 unemployed people willing to work for the same amount of money. Its simple demand supply relation, there are a lot of unemployed people and not enough jobs. So the wages are pretty low, will stay that way till there is some kind of equilibrium between demand and supply of workers. Companies from all around the world have their factories in India and China precisely because these countries provide very cheap laborers, which in turn decreases the production cost. This doesn't happen in countries like US and UK and other developed countries because there are not as many unemployed people moreover even lesser number of people willing to work for any wages give to them. This is same with the IT industry as well, its not that there is a grave shortage of Programmers in US, UK and other developed countries, but outsourcing the jobs to India and such countries gets the work done in a small fraction of money that they would have to pay had they given the job to someone in their own country.

Already happening in China, since a few years ago factories have been complaining that it is getting harder to recruit manufacturing workers. Now many factories are moving to the western provinces in China where workers are paid less than they would have been paid in Eastern provinces. Being close to home makes a job that pays lower wages more attractive.