Lenovo's Android update tracker has just been updated to give a time frame when specific devices will be receiving Marshmallow, if at all. To quickly clear the air - hugely popular devices like the K3 Note and A7000/A7000 Plus will indeed be graced with an Android M firmware, just not particularly soon.
While Android Marshmallow has been launched now, Google's Android N is likely to be launched in September 2016. so just blame it on the budget as phones are more likely to receive all updates based on how much they were priced when you paid for them. So its more likely that the phones that are more high end are likely tor receive all of Google's updates while phones that were low priced will take more time until hang on to the older versions fo Google. The yet-to-be-released Vibe S1 and the Vibe P1 that recently hit the shelves will get Android M in June 2016.
As of now, only few devices are confirmed for the Marshmallow update while the remaining phones will continue to run on Android 5.1 Lollipop.