Reason for the multiple sidelobes in an ILS glideslope signal
All directional antennas have sidelobes (and all real antennas are at least somewhat directional). I won't get into detail of the fundamental physics reasons behind that, as that's beyond the scope of the aviation stack exchange, but briefly it's related to diffraction and the Fourier relationship between the aperture distribution and the far-field radiation pattern.

Various design techniques can reduce the amplitude of the sidelobes, sometimes to the point where they're undetectable or at least insignificant. However, these all involve tradeoffs that compromise other performance parameters, size or cost, so if the sidelobes aren't problematic or can be easily avoided operationally, it's not worth trying to suppress them. Also, most glideslope antennas use passive phased-array techniques, and phased arrays are especially prone to sidelobe generation (called grating lobes in that case).