We have that answer somewhere in this site, if one Searches Rank and digs through the discussions. But off the top of my head, it goes something like this. Above the rank of Colonel the ancient rank system went to the General Ranks. Before we get into the General Ranks, we should look at Brigadier. The Brigadier was a full Colonel, who commanded a Brigade, with the Unit commanders in that Brigade being LCols. Brigadier was not a rank, but an appointment.
In the ancient rank system, the General ranks progressed through Sergeant-Major General, Lieutenant General (who was the right hand man of the General or his Lieutenant.), and then the top man, the General. Sergeant-Major General was quite a mouthful and over the centuries was shortened to Major General.
The rank of Brigadier General was created when someone decided that it would be nice to have a General command a Brigade, and so the appointment of Brigadier became a rank.