Why was J.J. Thomson wrong?

J.J. Thomson was “wrong” because he assumed that the positive charge of an atom was all one uniform blob of matter with the electrons moving in orbits inside the nucleus.
Thomson developed his model of the atom in 1904. North Americans would call it a “Christmas cake” model.
He assumed that the electrons were uniformly distributed and free to rotate in rings inside a sphere of positive charge.
These orbits were stabilized because, when an electron moved farther from the centre of the positive cloud, it felt a larger net inward force because caused by all the positive material inside its orbit.
Others (not Thomson) compared it to a British dessert called plum pudding, so his model came to be known as the plum pudding model.
Many North Americans would compare it to the raisins and nuts in a spherical Christmas cake.
Thomson’s model was correct at the time, because it explained all that scientists then knew about the atom.
The Japanese physicist Hantaro Nagaoka rejected Thomson’s model.
He suggested that the atom had a massive nucleus.
The electrons revolved around the nucleus, like the rings revolving around Saturn.
In 1911, Rutherford showed that Thomson’s model was “wrong”: the distribution of positive and negative particles was not uniform.
Rutherford showed that the atom contains a small, massive, positively charged nucleus.
He also agreed with Nagaoka that the electrons move in circular orbits outside the nucleus.
The Rutherford model was the best explanation of atomic structure for his time.
We now know that even the Rutherford model was not completely correct.
Models are not “wrong”. Rather, they are the best we have at the time. Scientists change them as new information becomes available.