Why do many people not use tabbed navigation (e.g. in an internet browser)?
Because a majority of the people are just too plain stupid.

In an international study by the OECD on adult competencies (PIAAC) that evaluated literacy, numeracy, and problem solving in technology-rich environments, it was found that only every third adult (in Germany: 36%) between 16 and 64 was able "to master more complex tasks such as navigating the internet".

Read that again. A more complex task such as navigating the internet. Only one third manages that. In an educated country like Germany where 77% of the population 14 and older use the internet.

This means that more than half the internet users don't know how to navigate the internet.

The study also found that


every eighth person does not know how to use a computer mouse
every fifth person only knows basic arithmetics or less
every sixth adult (!) has the reading ability of a ten year old child


I think tabbed browsing is a minor problem. The study makes me doubt the principles of democracy. How can people be allowed to vote, if about 15% of the adult (!) population "can at best read short texts with simple words and extract from these only very limited information"?

Source: http://www.oecd.org/site/piaac/