The World Wildlife Fund has more details on this info page about the threats, but in brief, it’s the usual suspects: netting, both deliberately and as by-catch; pollution and sedimentation; and dams and irrigation projects that fragment the dolphins’ habitat.*
The Ganges River dolphin is not alone, however: the Pink Amazon River dolphin is also endangered, and for the same reasons.
Dolphins are more vulnerable than some other aquatic creatures as they’re at the top of the food chain. Any pollution that gets ingested by prey is concentrated in the predators, and in general if the population of a prey species drops, the population of the predator species drops too, often alarmingly (If you have a mathematical turn of mind, you might want to check out the Lotka-Volterra equations, which describe predator-prey relationships. If you’re not mathematical, you might still want to look at the graphs on the page, as they show several examples that illustrate the relationships).
* If you’d like to help, go to the WWF page and click on one of the Share links at the bottom of the page.