To be accurate, this is not the reason many sites choose not to go with SSL for everything. The real reason is that most sites don't need to be SSL for everything.

I run a travel blogging site, where 99% of all pageviews are from random people off the internet reading people's trip reports and looking at photos. Encrypting all that traffic would do nothing except bog the site for everybody.

Every once in a great while (in terms of total traffic), somebody will log in and post something. That tiny moment could benefit from SSL, since chances are it's happening from a public internet cafe or wifi hotspot. That's the only time a user is actually vulnerable to this sort of attack, so that's when they need to be protected.

But when you look at the internet at a whole, the traffic fraction that needs protecting looks pretty much the same. When you're showing me pictures of cats with funny captions, please don't encrypt them before sending them to me just because you read something about security on HackerNews.