You didn't building in the conventional way, but I think this is fixable. The way I look at it there are three main things that are affected by high str/tackling scores:
1. Fewer missed tackles
2. Higher opponent moral loss
3. More forced fumbles.
It looks like your guy doesn't miss many tackles, that's good. The other two are harder to quantify. Forced fumbles are pretty rare regardless and there's no way of knowing the opponents moral loss.
The other X factor in making tackles is being in position to make the tackle in the first place. That is speed, agility, and vision. Definitely try to get those up to the 2nd or 3rd caps at least.
Also, looks like you can upgrade equipment, too.