TLDR: It’s probably because of your hosting provider.
I was doing a site walk-through with a client where I give them a tour of the back-end of their WordPress site as part of a client wrap-up. I started to show them where they could find the page revisions history on individual pages and I COULD NOT FIND IT. It was a live call being recorded and it was a little embarrassing. After the call ended and a couple of pages of Googling, I saw that I wasn’t crazy (at least in that regard). I couldn’t find it because it wasn’t there. Since the client hosted their website with WPEngine it was not available by default.
By default, WPEngine has Revision History Disabled.
As a managed host, WP Engine disables WordPress revisions and does not allow revisions to be enabled in the wp-config.php or php.ini.
We recommend using a separate editor to manage content prior to publishing. This protects your site and ensures positive server performance.
If you’d like revisions enabled on your website, [contact a] Support agent who can enable up to 3 revisions.
Enabling revisions is not a retroactive change and old revisions will continue to be removed automatically after 60 days.
It is possible to get page revisions option on a website hosted by WPEngine but you are limited to 3 revisions in an effort to maintain better site performance. Simply contact their support team and request that they enable it.
For more information check out WPEngine’s documentation on the matter.