Why is HV kept open in OC test of transformer?
By definition, an OC test of a transformer is an Open Circuit test.  Why would you connect the HV windings?  The open circuit test primarily is used to determine the core (magnetic) losses of the transformer, as with no load, you don't have to worry about copper (winding) losses.

For an SC test, the low voltage side is shorted in order to measure the impedance of the transformer.  The high voltage winding is connected to a variable voltage supply, and the supply is increased until the high voltage leg reads it's maximum current.  You don't need full rated voltage to achieve full current this way, and it makes it a simple matter to measure the transformers impedance, as well as the actual copper (winding) loss at full load since the magnetic structure of the transformer isn't driven into saturation (or anywhere near it, the magnetic losses are extremely low when a transformer is excited in this manner).