The descriptions above showing how to create fades for MIDI give a clue about why the corner-grip fade is not offered on MIDI clips.
 
MIDI fades require using one of the publicly defined parameters, volume or expression. A person might think these ought to be controlled by CC (continuous controller), leading to a solution whereby Sonar just scales the parameter down to zero or up from zero for fade-out or -in based on the user's desired set value. Unfortunately it is not always possible for Sonar to know the user's set value. It could be hidden in a synth preset or a sysex message, instead of being set in a MIDI CC or envelope that Sonar can follow. Therefore it is left to the user to decide how to use the CCs.
 
Audio fades do not require using any publicly defined parameter. Sonar can just scale audio samples between 0% and 100% to compute fades, and offer a choice of curves as well.