He could refract, reflect, diffract the waves. Everything was fitting in just as predicted by Maxwell's equations. Light carries energy, and some of that energy could knock into electrons. It's like a tidal wave jiggles pebbles exactly the same as little teeny waves do. Even if the light is very dim.
Waves are generally spatially big, and deposit their energy over large areas.
To knock out an electron, you need to give it lots of energy. This maximum increases linearly with the frequency. Namely, despite all the successes and experimental evidence that light is a wave, this seems irreconcilable with the photoelectric effect.