So far, a definite reason for this has not been established. All we have are theories, assumptions and guesses.

The most credible theory is the one that says that natural disasters were the reason for the decline of the IVC. Several layers of silt at Mohenjodaro point to repeated flooding of the city. Some argue that the flood occured due to tectonic movement at a place called Sehwan, 90 miles downstream from Mohenjodaro may have caused the formation of a gigantic natural dam turning the area around Mohenjodaro into a huge lake. However such tectonic movement cannot satisfactorily explain repeated flooding.

While Mohenjodaro may have been affected by severe flooding, the sites on the Ghaggar-Hakra valley was affected by gradual desiccation. The Sutlej or the Yamuna once flowed into Ghaggar however tectonic shifts led to river capture. Either the Yamuna joined the Ganga system or the Sutlej was captured by the Indus.

Other reasons include, climate change, foreign invasion.

Personally, another reason that I believe contributed to the decline was that a strong unifying element, the Harappan state, came to an end. This is evidenced by the stop in use of standardised weights and measurements and switching to local non-standard weights.

We'll never know for sure...