The newer generation Fire tablets run a newer Fire OS version (which is based on a newer Android version)… perhaps it has something to do with that. 
Based on the data below, it looks like the 7th generation Fire HD 8 can be upgraded to Fire OS 5.6 as the newest version (based on Android 5) whereas the 8th and 10th generation Fire HD 8 devices can be upgraded to Fire OS 7.3 (based on Android 9).
Fire HD 8 Generations and OS
From the Fire HD Wikipedia entry:
| Generation | 7th (2017) | 8th (2018) | 10th (2020) |
|---|---|---|---|
| Model | Fire HD 8 | Fire HD 8 | Fire HD 8 |
| Model Number | KFDOWI | KFSUWI | KFONWI |
| Release date | 6/7/2017 | 10/11/2017 | 7/12/1905 |
| Release Time Fire OS | 5.3.3 | 5.3.5 | 7.3.1.3 |
| Latest Fire OS | 5.6.8.0 | 7.3.1.7 | 7.3.1.7 |
Fire OS Versions
From the Fire OS Wikipedia entry:
| Fire OS version | derived from Android version |
|---|---|
| 3 | 4.2.2 |
| 4 | 4.4.2 |
| 4.5.1 | 4.4.3 |
| 5 | 5.1 |
| 6 | 7.1.2 |
| 7 | 9 Pie |