Why is this?
I think it boils down to two reasons - most people of that age don't feel the need to get online, and/or they view computers as things that they would not understand.
I have tried to teach an 86 year-old man how to use email on a laptop, and he just couldn't manage it. In the end we both agreed to call it a day - he carried on phoning his family instead.
I think that we have to accept that there are aspects of technology that many (but obviously not all) people over 75 might generally struggle to grasp. It's a passing problem though, and will largely disappear as the current generation of middle-aged online users reach old age. They'll have spent decades online, and they'll carry on until the time comes for their last power-down.