Why Was the Franchise Extended?

I'm thankful that I live in a democracy, but in the not-so-distant past, the electoral franchise was limited to males with a certain amount of wealth. Why did elites extend the franchise to females and the poor? Toke Aidt and his colleagues have a VOX column on this issue. Here is the executive summary of their column:

Some theories suggest that the threat of revolution plays a pivotal role in democratisation. This column provides new evidence in support of this hypothesis. The authors use democratic transitions from Europe in the 19th century, Africa at the turn at the 20th century, and the Great Reform Act of 1832 in Great Britain. They find that credible threats of revolution have systematically triggered pre-emptive democratic reforms throughout history.

I've contributed to this literature with a paper looking at the 1867 extension of the franchise in the UK. The paper is available here. I did not find much evidence to support a threat-of-revolution explanation for the extension of the franchise in 1867.