As you know, 1v1 ladder pools are cumulative, in the sense that you get all maps of your bracket and lower. Isis is put in the pool for the <200 bracket. This means that the <200 players have a 1 in 4 chance of playing their game on this map. The other three maps also being easy 5x5 maps. For a 1800 player like yourself you will get games from a pool of 20 maps. This means you have 1 in 20 chance to get Isis. The map is of course less suitable for the higher bracket, and therefore there is less chance you will play on it.
As for excluding 5x5 in ladder.. Opinions differ a bit on this. However, its definitely true that interest in playing 5x5 maps is inversely correlated with rating, meaning higher rated players are less likely to enjoy playing on a 5x5. But there are quite a few exceptions. That's why there was decided to increase the probability to play on a 5x5 map when you are lower rated and decrease it when you are higher rated. However even when you are 2k+, you will still play 1/5th of your games on a 5x5..
Excluding 5x5 maps from high level 1v1 ladder altogether would necessarily mean that we cant use the cumulative system anymore, because new and low rated players need these maps to ease into the game. There have been some discussions about this in the matchmaker team, but there was no consensus on changing to a non cumulative system like tmm.