Probably because if people would put a 100 diffrent things in their cart and then purchase them at once all the stuff wouldn't go in their inventory and just dissapear. Also if a group of people kept spam buying a large amount of stuff then it would most likely slow the server itself down.
I wouldn't recommend putting many of the same items in your inventory at once.
Reason being is that you may get the "Count restriction" kick message which basically kicks you from the server as soon you try to enter a spawn location because you have too many of the same items in your inventory (for example 20 FAK's).
It's mainly to reduce server temporary storage however it also goes along with what @DairyleaPP mentioned on the "Count restriction" kick of where once you receive numerous items over a large count it kicks you to prevent duping as usually duping occurs in large quantities. Therefore this new shop system is much more effective and simpler than before, Although i disapprove that you can't sell weapons as cops :V