Because,In quantum world every particle behave like the wave or other mean they are wave.Or more explicitly explanation:
Particles in the nature,they behave according to the which experiment is done.
When a electron collide another particle,you can imagine that like the billiard ball.But i doble split experiment,particle behave like a wave.They create interferrece pattern or other behaviors of wavenature.
De broglie suggested that,a wavelength escort the every particle but scale of large and massive body you can not observe it.