Why portable split air conditioners not common? What's the catch?
Not so much of a catch but houses in UK are so rarely specified with AC (only needed 3 days a year .. :) ) that there is a (small) market for the portable ones.

Houses in US have AC included in the build so often that it is almost standard - older housing stock may get upgrades.

Remember the refrigeration cycle used is only relatively recent compared to the time we have been building houses. 

Thise portable units can work very nicely but making sure that the air volume is controlled is important otherwise you can end up trying to cool the atmosphere... ie windows shut etc