Why can't pyruvate be used directly in the Krebs cycle?
Oxidative decarboxylation of pyruvate to acetyl-CoA by pyruvate dehydrogenase is coupled with reduction of NAD+, which can feed into the electron transport chain. 
CoASH activates the acetate: hydrolysis of the thioester is thermodynamically favourable and drives the reaction catalyzed by citrate synthase.
Conversion of pyruvate to acetyl-CoA commits it to a subset of possible fates (including the tricarboxylic acid cycle) and is therefore a key regulatory step.


You can read more about this in most introductory biochemistry texts, like this one. 

Also note that pyruvate actually can enter the TCA cycle via carboxylation by pyruvate carboxylase to oxaloacetate. This reaction requires energy in the form of ATP and is used to regenerate TCA cycle intermediates as they are siphoned off for anabolic pathways.