The lazy answer is that >95% of users have images and JavaScript enabled, and of that 95%, those who have a slow connection are getting the same experience across a number of websites, so they probably won't be turned off by a slow site.
The better answer is that unless you think about it from the start, providing a no-JS fallback can be hard to do well and may require a fair bit of re-architecturing of your website or web app - something that you probably won't bother doing to do for a single digit percentage of your visitors (text-only support probably has a lower priority than supporting <IE9 and Opera Mini). Finding out that images won't work is also pretty hard - they're no way that I know of finding out if images are enabled or not without using JavaScript, which is probably disabled as well.
I've actually been creating a reasonably complex web-based application recently, and for fun every few weeks I test it using Lynx[0]. Technically, using forms and very basic CSS results in a fairly usable service. I managed to get a working CSS3-only (no JS) fallback for tabs, some toggle buttons and some other UI goodies. The only problem is that I end up duplicating a lot of stuff on the backend of the app (because I have to deal with both form submissions AND ajax), and there are a lot of things that you can only really do with JavaScript and images (e.g. games, most interactive stuff without reloads, anything to do with images obviously).
EDIT: regarding advertising, rarely do (effective) advertisers online want the largest audience specifically, they want the most engaged audience. Is this no-JS and no-images user interested in downloading an app or a pop album? Probably not, so it's not worth putting in the effort to make them see your ads.
Comments
The lazy answer is that >95% of users have images and JavaScript enabled, and of that 95%, those who have a slow connection are getting the same experience across a number of websites, so they probably won't be turned off by a slow site.
The better answer is that unless you think about it from the start, providing a no-JS fallback can be hard to do well and may require a fair bit of re-architecturing of your website or web app - something that you probably won't bother doing to do for a single digit percentage of your visitors (text-only support probably has a lower priority than supporting <IE9 and Opera Mini). Finding out that images won't work is also pretty hard - they're no way that I know of finding out if images are enabled or not without using JavaScript, which is probably disabled as well.
I've actually been creating a reasonably complex web-based application recently, and for fun every few weeks I test it using Lynx[0]. Technically, using forms and very basic CSS results in a fairly usable service. I managed to get a working CSS3-only (no JS) fallback for tabs, some toggle buttons and some other UI goodies. The only problem is that I end up duplicating a lot of stuff on the backend of the app (because I have to deal with both form submissions AND ajax), and there are a lot of things that you can only really do with JavaScript and images (e.g. games, most interactive stuff without reloads, anything to do with images obviously).
EDIT: regarding advertising, rarely do (effective) advertisers online want the largest audience specifically, they want the most engaged audience. Is this no-JS and no-images user interested in downloading an app or a pop album? Probably not, so it's not worth putting in the effort to make them see your ads.
[0] http://lynx.isc.org/