Back when Motorola was owned by Google, the company was known for fast software updates. The 2014 Moto X was the first handset to receive an update to Android 5.0, and you could rely on phones like the Moto G getting prompt updates as well.
Under Lenovo, things have changed. While Motorola still ships smartphones with near-stock versions of Android on-board, the company isn't willing to provide frequent software updates. Case in point: Motorola will not provide monthly security updates to their portfolio of devices, including the flagship Moto Z.
According to Motorola, it's too difficult to keep their smartphones secure through monthly updates. Instead, security updates will be bundled into scheduled maintenance releases, which are much less frequent.
Here is Motorola's official statement on the matter:
Motorola understands that keeping phones up to date with Android security patches is important to our customers. We strive to push security patches as quickly as possible. However, because of the amount of testing and approvals that are necessary to deploy them, it's difficult to do this on a monthly basis for all our devices. It is often most efficient for us to bundle security updates in a scheduled Maintenance Release (MR) or OS upgrade.
As we previously stated, Moto Z Droid Edition will receive Android Security Bulletins. Moto G4 will also receive them.
Providing users with timely security patches is key to keeping them secure, which is why Google is providing official Android security updates on a monthly basis. Some of the best high-end phones on the market are being updated when these patches are released, such as Samsung's Galaxy S7 series and Google's Nexus line-up, but the Moto Z and Moto G won't be among them.