Why has nobody unified the four fundamental forces? What are the key differences between them?
The problem with unifying the fundamental force is that quantum mechanics and classical theories are incompatible.
Electricity and magnetism were considered completely separate until they were unified in the late 19th century into electromagnetism.
The electromagnetic force and the weak interaction were unified into the electroweak theory on the mid 20th century.
The strong force which is actually a residual effect of the colour force which binds quarks is a quantum theory. It is compatible with the electroweak theory in that they are both quantum theories and have similarities.
A grand unified theory which unites the electroweak theory and the colour force should be possible.
Unifying the other forces with gravity is a problem. Gravity is a classical theory. It describes gravity not as a force but a consequence of energy curving space time. Describing gravity in terms of quantum theory is very problematic.