What's so bad about sitting at Har Sinai?

Why does the parsha refer to Mount Sinai as 'this mountain' when he orders the Jews to leave?

Tuvia and Yitzi ,

Tuvia and Yitzi
Tuvia and Yitzi

This week we start Sefer Dvarim, Moshe begins his final speech to Am Yisrael, telling over what happened in the desert and preparing them for entering the Land of Israel.

For some reason, Moshe doesn't begin with telling the story of Yetziat Mitzraym (the Exodus) or from Matan Torah (giving of the Torah), but rather from when Am Yisrael were told to leave Mount Sinai.

But even more surprising is the harsh language Moshe uses describing the commandment to leave Har Sinai and move on "Enough sitting at this mountain!!".

Why refer to Har Sinai the place they receive Torah like that?? And why does Moshe choose to begin his speech from that point in time?