You must look at the history of it. Disabling images was <i>very</i> important when accessing web over a 9600 bps modem. Disabling Javascript overall was also convenient as practically no useful content was brought by Javascript but a lot of annoyance, and some exploits always existed.<p>I still browse with JS disabled by default, however I turn on JS for the selected sites, only if I'd believe that they are worth it. Opera allows me to be selective, it has Javascript block configurable per site.<p>Why not CSS? Well, Opera can set a user css per site too. It's very convenient for some of the sites that are unreadable for me by default (i.e. small white letters on the black background or even picture background). It still can't explicitly disable HTML5 videos per site, AFAIK.
For security reasons? I feel like that too obvious an answer. Maybe I'm missing something.<p>Javascript can be potentially harmful because it's code running on your system, albeit sandboxed to some degree. You can also embed code in images, but I don't think that's why people disable images. It's probably either because third-party sites can track your web usage through images or simply because they want a simpler browsing experience.