How do I figure out which version of Android is supported on my device? I am a Pixel 2 that is running Android version 9.

I also have a much older Nexus tablet.  I get an update on my tablet almost every day but it is only for Androids version 5.1++.

Why can't I upgrade to version 9 on my tablet too?

Is there some sort of hardware limitation and if so, how do I figure out what the latest supported version for my device is?
It is not a hardware issue but rather a resource management issue in maintaining a device. Which is evident through custom ROM's they are able to maintain updating Android devices to the current Android version.

Since your device's are maintain by Google you have you recieve the most dependable update schedule with a straightforward device update policy.

Nexus policy:


  Nexus devices get the latest version of Android directly from Google.
  
  If you bought your device from the Google Store, updates will typically reach your device within 2 weeks. If you bought your device elsewhere, updates can take longer.
  
  Nexus devices get Android version updates for at least 2 years from when the device first became available on the Google Store. After 2 years, we can't guarantee more updates.
  
  Security updates
  Nexus devices get updates for security issues documented in our Public Android Security Bulletins.
  
  Nexus devices get security updates for at least 3 years from when the device first became available on the Google Store, or at least 18 months from when the Google Store last sold the device, whichever is longer. After that, we can't guarantee more updates.


({Device then end of update guarantee date}

The first date is: 

No guaranteed Android version updates after. 

The second date is: 

No guaranteed security updates after.

The third date is:

No guaranteed telephone or online support after)


  Nexus 6P
  
  September 2017 - 
  November 2018 - 
  November 2018
  
  Nexus 5X
  
  September 2018 - 
  
  November 2018 - 
  November 2018
  
  Nexus 9
  
  October 2016 - 
  October 2017 - 
  October 2017
  
  Nexus 6
  
  October 2016 - 
  October 2016 - 
  October 2017
  
  Nexus 5
  
  October 2015 - 
  October 2016 - 
  October 2016
  
  Nexus 7 (2013)
  
  July 2015 - 
  August 2016 -
  August 2016
  
  Nexus 4
  
  November 2014 - 
  November 2015 - 
  November 2015
  
  Nexus 10
  
  November 2014 - 
  November 2015 -
  November 2015
  
  Nexus 7 (2012)
  
  June 2014 - 
  June 2015 - 
  June 2015


How do I figure out which version of Android is supported on my device?

One of the benefits of owning a Nexus and/or Pixel is the fact that it is maintained by Google and all firmware is freely available. You can verify what official Android version you device can be updated too by simply go to Googles developer factory image repository.

Why can't I upgrade to version 9 on my tablet too?

After your nexus stops recieving official Android updates it does not mean your device cannot be upgraded to a newer android version. The great thing about open source is anyone can utilize the the source code to develope updated Android version devices that are no longer officially supported by the OEM.

The Nexus is a very developer friendly device. The possibility of getting the up to date Android version are higher through a custom ROM. 
For example I have a nexus 7 (2013) Flo. I am running a custom Android 8.1 ROM. Iin the process compiling and  updating things. Trying getting things to hopefully work with android 9 pie for my nexus.