Because Aran, If you know anything about nuclear physics, which by your comment you don't. It's like pushing a car. It takes a lot of time and effort to get the car to start to move but very little effort to keep it moving and make it move faster. The hard part is starting the enrichment process. Upping the enrichment level is the easy part if you have the correct equipment.
Official: Top Burundi general assassinated in the capital (AP)