When new hardware products are released, it's usual for them to only support the newest releases of firmware - and this is the case with the MX75 (and the MX85, 95, 105, released at the same time).
Release 16 of firmware is therefore considered Stable specifically for those platforms. You may see upgrades automatically scheduled for your MX75 to newer versions of 16.x as they become available (these would also be Stable, for your MX75) but should not see similar for 17.x until it at least becomes available as a Release Candidate, in a future iteration.
MX75 will not run 15.xx firmware
More info on such dependencies is available here: https://documentation.meraki.com/General_Administration/Firmware_Upgrades/Product_Firmware_Version_R...