Why are vectors used for sequencing rather than speed control?
Let's say you have two aircraft that will reach the same point in space at the same time, and your job is to keep them three miles apart.  One way to do it is to calculate how many knots the second aircraft should reduce their speed so that at the time the first aircraft reaches the point, the second aircraft will have fallen three miles in trail.

Unfortunately for you, this means that you have to work a time-speed-distance-wind problem for every single aircraft that you, as a controller, handle.

Alternatively, you pick a point three miles away and tell the second aircraft to fly there.

Air traffic controllers are human, and humans are better at angles than we are at numbers.  The simplest thing for controllers is usually the best thing - they've got lots on their mind already without making them do math too!  This might change in the NEXTGEN air traffic network (where everyone has a GPS position and speed readout right there on the screen) but for now, easy is better than hard.