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Why did light tanks armed only with machine guns become less useful during the 1930s?

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Kuwait: U.S. 1st Armored Division M1A1 Abrams tanks — tank M1A1 Abrams main battle tanks of the U.S. 1st Armored Division moving across the desert in northern Kuwait during the Persian Gulf War, February 1991.

During the 1930s, light tanks armed only with machine guns became increasingly obsolete as the proliferation of tanks among various nations necessitated vehicles capable of engaging other armored units.1

As tank production expanded globally, the tactical value of these lightly armed vehicles diminished. They were progressively displaced by heavier models equipped with 37- to 47-mm guns specifically designed for anti-tank combat. Furthermore, the emergence of dedicated antitank guns in the 1930s—typically in the 20- to 40-mm range—meant that tanks required thicker armor to survive on the battlefield.123

Experience during the Spanish Civil War demonstrated that even relatively mobile tanks were often too lightly armored to provide effective infantry support or survive exposure to modern anti-tank weaponry. Consequently, military focus shifted toward tanks with more substantial armor and, eventually, more powerful 75- to 76-mm guns to maintain combat effectiveness.1