What is the reason Jira's UI keeps changing in a very significant way every year?

[Michael Gerzon]

What is driving the significant UI changes that Jira goes through ?
seems like the amount of users that doesn't like the new Jira look is large. 
First Jira was updated from the old Jira layout to a new one. Then a second change came to impact how individual ticket looks like. 
Why to do such drastic changes to a community that is already used to work with a product for many years ? 

[Nic Brough -Adaptavist-]

It's in the interests of moving forward and never becoming tired or ordinary, and evolving the products.

Trying to look on the bright side, for me, it encourages me to re-evaluate things I might be taking for granted.  Each change makes me question if it's a good or bad thing.  I'm fully aware that I'm a "stick in the mud" type person and probably resist change more than the mean-average person does, so I try to see the changes as a something that pushes me to think again. 

But I completely understand "stop messing with stuff, I just want to use it as it is and not have to learn new stuff every few days".  And, I still don't think the left-hand layout or the new issue view are improvements overall, there's more counter-intuitive stuff and scrabbling around to find things that should be obvious than before, although there are some things that work really well.