A landslide is a non-coherent change, because the backscatter characteristics of a surface (all involved pixels) mostly change. InSAR is only able to measure elevation differences of coherent processes. Additionally, surface differences of the size of the wavelength (Sentinel-1 has 5 cm) can be observed at most per image pair. Anything larger (usually the case for landslides) cannot be measured.
See mengdahl’s comment here