A testimony to the severe fragmentation in the Android space, Marshmallow is still the most popular version of Android. Google released android 6.0 back in 2015. That is a really long time ago.
Google periodically updates the OS distribution charts. In the latest update dated 08 January Marshmallow has a market share of 28,6. This is more than the 26,6% commanded by both versions of Nougat. Most of those devices were released 2 years ago. For most manufacturers that means the devices are now out of their support lifetime.
Fragmentation has long plagued Android. Google promises to improve the situation with Android 8, Oreo. However, up to now, the latest version of Android has a market share of just 0.7%. This means even though updates will faster, the changes will only apply only to those on Oreo. And it’s going to take a while for that to happen, with manufacturers still releasing phones on Nougat. Maybe Google should put restrictions on that as well.
Android manufacturers also don’t take part in the battery gate, unlike Apple. Android users tend to not see a value in upgrading their phones. Phone capabilities have also plateaued. An Android phone from 2015 is still plenty capable for most users. And a camera from that era? Still good enough for everyone.
Google hardware partners are reluctant when it comes to updating the operating systems of their phones. When Google releases a new version each device manufacturer has to test it on their own hardware. This does not go down well with them, considering you have already pay them.
In terms of capabilities, the OS is still okay as well. Google can bring some newer functionality through updates to Google Play Services. This brings new capabilities to older versions, encouraging users to remain on those for longer. Still, the sweet features of the new operating systems cannot be backported. And no one will know about those.