It's the temperature, a lot of chemistry doesn't work at higher temperatures because everything is too unstable. There is simply too much energy messing things up. This is why having a surface temperature that allows for liquid water to be present is such a good indicator for life. A lot of chemistry for life as we know it works at liquid water temperatures and water does play a big part as well.

The pressure would be less of an issue, there is plenty of life on Earth that thrives at huge pressures.

I'm pretty sure life on Earth evolved at the surface (or even in the atmosphere, it is thought lightning plays a part) and only adapted to use the vents later on. I'm not sure life could get started at those volcanic vents.