Nik's response:
"there never was a requirement to finish the whole torah in one year... a cycle used to be done every three years... the question is better asked why doesn't simchat torah and shavout coincide...?
the answer to this is also simple... because shavout is when we began studying torah and is a time for newness and starting things not finishing them... and we are never done with learning torah... that is why after we finish the whole torah on simchat torah we immediately begin again from the beginning with sefer (and parshat) bereshit... because torah never ends...
also... rosh hashan is not a time of celebration the same as simchat torah... true there is joy and we do enjoy the yom tov... but we are standing in judgement for our very lives here in this world and in the world to come... and thus we have other things to do to serve g-d on this day and we need to concentrate exclusively on this... blowing the shofar and davening the musaf prayer where we re-crown G-d king over the world and over us each year... it is a time for renewal not a time to sing and dance... the rejoicing over the torah comes later at the end of the tshuvah process which is repentance from fear (rosh hashan and yom kippur) and repentance from love (succot)... then we can rejoice in full confidence that Hashem has accepted our penitence and heard our tefilot and will bless us with a sweet new year and another full year of life... please G-d... nik. out... "