The answer is pretty straightforward: the only examples of the Indus Valley Script have been very short, generally on personal seals and the like. No bilingual inscriptions in Indus script and another have ever been found (no “Rosetta Stone”) and indeed, no lengthy inscriptions at all of any kind.
Without more, decipherment is (currently) not possible, as there is no context for the symbols, or even enough symbols to speculate on how they relate to each other in sequence. No “grammar” or any other solid clues as to how the language works. Some attempts have been made using Markov analysis to understand the structure of the language:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2721819/
This paper concludes that the script was, in fact, a writing system, and raises the interesting possibility that the script was used in different places to record different languages.
Archaeologists have long speculated that the reason for this lack of evidence is that the script was typically written on perishable media, long since disintegrated. Later civilizations in India, for example, used palm leaves to write on - the Pali canon of Buddhism was (and still is) written on palm leaves. Such media would be fortunate indeed to survive centuries of abandonment. Personal seals, of course, were carved of harder materials and so many have survived.
Some scholars doubt this hypothesis and have proposed that the symbols found are not, in fact, “writing” at all:
As noted above, the Markov modelling would tend to suggest that this conclusion is incorrect and that the Indus script is, in fact, “writing”. However, absent new discoveries, actual decipherment appears unlikely.
Well as stated, it's been tough even classifying the glyphs as a script, and for another, deciphering the texts of previously lost civilizations is next to impossible. The only reason why we know how to read Egyptian Hieroglyphs is because of sheer luck. And that's not hyperbole. The French found a stele in Egypt that happened to have Hieroglyphs alongside Greek, a text we already knew. The reason the Greek was there is a testament to the randomness of history. The dynasty that ruled over Egypt at the time (the ptolemies) just happened to be Greek. Obviously it took some really smart folks to actually get us to the endzone in terms of deciphering the text, but the Rosetta Stone itself was a critical part of the process. I don't think we'll have much luck at deciphering the Indus Glyphs unless some Rosetta Stone-esque object is unearthed. That being said, I really wish we did know more about their culture. The people of the Indus valley were nearly unparalleled in the annals of history in terms of their advanced civilization. Just my two cents.