Because tax evasion is so rampant in Greece, the IMF reasons (correctly) that tax increases wouldn't raise anywhere remotely as much revenue as Syriza claims they would.
My understanding (which is purely derived from reading news sites FWIW) is that IMF view heavy tax increases as likely to reduce economic activity in Greece (i.e. companies will go elsewhere to do business) and that this would have a bad effect on tax receipts, so would be in essence, self-defeating...