Because the data has to travel all the way from your Slingbox to where you are, going through many potential bottlenecks.
The speeds you quote are just 1) the speed from the Slingbox to your ISP in the US, and 2) the speed from where you are watching in Uruguay to your ISP Uruguay.
There could be any number of hops in between with their own limits slowing down the data.
Plus, there is something called 'latency' which describes how long it takes a packet of data to go from one point to the other and this rises with distance. As it gets higher it imposes a theoretical limit on the bitrate you can achieve.