Let me see,
1) Cost; many low income and pensioner households cannot afford to buy a computer nor can they afford the cost of broadband.
2) Complexity; Computers are not things you can just plug in and switch on like a washing machine. They need to be set up, all sorts of adjustments and changes need to be made to them before they are ready to use. Although I am a high level user of computers and have been using them for nearly 35 years I still rely on DH or DD. to set my computer up and do all the trouble shooting for me. I get a magazine called Computeractive aimed at people like me and the very basic nature of many of the questions on it. 'How do I make folders?', 'How do I delete a file?' make it clear how many computer owners really have no idea how to use a computer even though they possess one.
3) Illiteracy; There are a significant proportion of the population who are functionally illiterate. They may be dyslexic or have managed to get through school with out grasping more than a hazy idea about reading but you need to be literate to use a computer.
4) To put it bluntly their are a significant group of people who may not have learning problems but are not far above that level, they cannot grasp how to use computers or think through even the simplest problems.
5) People, sometimes in the above two groups, who have chaotic and disorganised lives, possibly addictions, that make the learning of any skill, let alone the discipline or money needed to buy a computer, broadband line and use it effectively. It is not the ability to utilise computers that is lacking but the personal organisation skills
6) Ther are many older people, not with dementia, but with disabilities and medical problems that make life very diificult. Constant pain robs you of the ability to concentrate on anything, medication that smudges the senses again makes concentration and learning difficult. Some people just have 'tired minds' and do not want to know, some are just anti any technology, anything modern etc etc
Hows that for a start