Pride and Prejudice shows these landed gentry of Victorian Era England as very wealthy and powerful people in England. Why aren't they so prominent today? Did their fortunes die out? Did they lose power but retain their estates? Do they still exist, just in a much quieter way?
A minor quibble: Queen Victoria came to the throne on 20 June 1837 but Pride and Prejudice was published in 1813. I gather it would be called Regency or Georgian (a longer period).
Not an expert but it was basically a process of them losing political power and then losing out economically due to competition from the colonies. A lot of them became ''land rich'' which basically meant they had a lot of land, but no money. But they were still pretty prominent until after WW2 when the UK introduced inheritance tax which meant that a good chunk of them could no longer afford to keep their estates, so they sold or destroyed them and pretty much began living like ordinary people.
They still do exist, a few still own estates and live in manors etc. They just have no real power outside of owning a lot of land, which they typically turn into parks for the local communities. In my experience, in many parts of the British countryside they're popular in their communities for using their land in this way