Hmm after my LG G4 I know why, never got updated, the last update it did get severly compromised battery life. Though I will say the hardware was decentish, though the Snapdragon 808 was a bit slow.
The last sony, similar issue, got one update and never saw another one. Samsung on the other hand got at least 2. So far my Nokia 8 has had monthly, that's how OEMs should do it. But I know my Nokia will stop soon :-(