Listen to the late Queen speaking in 1952 and then in the years before she died. There's a very interesting article and recording tracking it that I often use with my students.
Her accent is unrecognisable.
My accent has changed over the years since moving around. My daughter's accent has changed in the two years since she's been away at university.
If you watch the David Beckham documentary on Netflix, his accent changed as well.
Unless you have never moved from your home town, and only associated with the same people you associated with in your childhood, everyone's voice changes in some way.
Technically- because we subconsciously mimic what sounds are around us.
Sociolinguistically- we want to fit in with the group around us. That's why people's voices can get posher, or in the case of HRH, less so.
William and Kate are also examples. Her voice got posher with age- his got less so. Hers was a result of elocution lessons (probably) and a desire to fit in with the family she married into. His is now much less posh than hers, because he's got a solid position within the hierarchy and doesn't need to prove himself to them. But he does to the crowds. So he's been trained to speak like them.
It's fascinating.