Different codecs support different sorts of things. Some have more options than other. Plus, the engineers at Sony Media Software choose what to reveal and what to hide with some codecs. And even then, some codecs support some things and not other, such as alpha channel (RGBA, RGB+, 32-bit color) transparency.
One reason is that you have to check something to open up those options. For instance, if you have Constant Bitrate selected, you cannot change options under variable bitrate. Ensure you have the option selected that is attached to the greyed out options.