Why did J. J. Thomson reason that electrons must be a part of the atoms of all element

Same particles (electrons) were emitted even after changing the cathode material.

Explanation:

In his famous experiment, Thompson tested the properties of atomic particles. He used a cathode ray tube to apply voltage on the cathode. This generated beam of electrons, also called cathode rays. He bombarded the rays on phosphorus on the other end of the tube, to observe the pathway it took.

When he noticed the deflection of cathode rays when it passes through the electric and magnetic field, he repeated the experiment by changing the cathode material. To his surprise, rays emitted from all the materials exhibited the same behavior.

He concluded that these rays comprising of electrons, are a fundamental part of atoms of every element.