So, while i was in the army, i noticed that the unit numbers (66th MI BDE, 101st INF, etc.) seemed to have random numbers. For example, we have had 1st-9th INF DIV, skipped 10th, there was 11th, then skip to 17th, then 19th, then 23rd-49th, skip 50th, and so on.
Of all the divisions the army has had, it seems the numbers we still have are arbitrarily. We have 1st-4th, then skip to the 25th (for active service). What happened, why were the ones that were chosen to be kept, kept?
The 10th INF division was converted to the 10 Mtn during WWII and then deactivated and re-activated several times since then. The final time during the Reagan administration.
The rest of the missing division numbers are either deactivated or National Guard Formations. The various non-divisional were numbered at the time of their establishment and then deactivated or reactivated as the needs of the army changed.
The reason behind which divisions were kept active has to do mostly with their history and role. The US Army's Airborne Division is going to be the 82nd. The 1st Infantry has a proud history and hench it will remain active. Sorry, there is not a more logical answer for you.
There is a 10th Infantry Division - the Tenth Mountain.
But to answer - a lot of units that previously existed have been deactivated. A whole lot of divisions that were formed for WWII weren't deemed necessary after the war's end, and were dissolved. This applies as well to Regiments and Brigades, since at that level in some cases you're talking about units that were formed up sort of haphazardly during the Civil War, and then occasionally reformed and then disbanded later.
As to why which units were disbanded, it had to to do with a combination of combat record and politics. When WWII ended, which unit was more likely to be kept active - the legendary and specialized (skill-wise) 101st, or the virtually unknown 106th?