Why don't people like to pick/use Tenacity over Determination?

[Reinhardt_Azureheim]

You get the same amount of benefit. There is no diminishing return.

20% Mitigation = 125% EHP.
36% Mitigation = 156.25% EHP (A 25% gain from the previous)
51.2% Mitigation = 195.3% EHP (a 25% gain from the previous)

While your theory is correct and relative to each previous step it is a steady increase of +25% eHP (and relative to the original it grows exponentially), the reason we call it "diminishing returns" has nothing to do with eHP; the core of the explanation was "you can't reach -100% damage taken stacking %-based cooldowns because of the calculation being multiplicative, not additive", i.e. 20% + 20% =/= 40% but 36%.

Apart from that, you mistyped the 3rd mitigation. That should be 48.8% mitigation (51.2% remaining) to give 195.31% effective HP; 51.2% mitigation would be 200%+ eHP.