Nagarjuna and the Schools
I can tell you why the Theravada school does not pay attention to Nagarjuna's works:


Parable of the Poisoned Arrow
Parable of the Simsapa Leaves
The Discourse on the Unconjecturables
The statement by the Buddha that he did not hide anything in a closed fist.
Discourse on The All


Basically, this means that from the perspective of the Theravada school, everything that the Buddha had taught in the Pali Canon is sufficient. Nothing more is needed. Anything else is simply icing on the cake.

Personally, I have tried to understand Nagarjuna's Madhyamaka emptiness and tried to link it back to Theravada emptiness in this question. I appreciate the concept of Madhyamaka emptiness.

I don't think Nagarjuna is in conflict with the Buddha's original teachings (in the Pali suttas and Mahayana Agamas). But at the same, I don't think he introduced something new to improve the Buddha's original teachings.

One important contribution of Nagarjuna is the use of methodical philosophical analysis to defend Buddhism against attacks by Hindu and other non-Buddhist Indian philosophers. See this answer.