Why is mRNA the least stable compared to rRNA and tRNA?

Generally its longer, and has less secondary structure.

As its longer it is more prone to shearing between the nucleotides. Further there are fewer secondary interactions between the nucleotides with in the same strand, less Hydrogen bonds etc. thus providing an easy target for many RNAse enzymes. Its also targeted and generally meant to be less stable as it is beneficial for the cell to be able to change its mRNA output quickly to suit changing conditions.