zlacker

It is primarily due to the fact that changes in altitude with regard to achieving orbital synchronization take time. This is also related to the concept of a launch window. Today's launch had an instantaneous launch window, required to reach all mission objectives, and for Dragon to achieve its precise window of opportunity in space. This means that the launch had to occur at an exact moment, down to the exact second of accuracy.

For a more exhaustive understanding, further explanation and exploration, you will enjoy the following:

https://en.wikipedia.org/wiki/Orbital_mechanics

https://en.wikipedia.org/wiki/Orbital_speed

https://en.wikipedia.org/wiki/Orbital_plane_%28astronomy%29

https://en.wikipedia.org/wiki/Orbit_phasing

https://en.wikipedia.org/wiki/Hohmann_transfer_orbit

https://en.wikipedia.org/wiki/Space_rendezvous

https://en.wikipedia.org/wiki/Docking_and_berthing_of_spacec...

https://en.wikipedia.org/wiki/Launch_window

>>Arjuna+(OP)
Obligatory: Then play some KSP.

>>stcred+Ca
Or play KSP for a while first, then read the links and understand much more of them instantly.

>>MPSimm+Wn
It's an obligatory thing to say, but combining experience tinkering with these things (KSP) with the reading is the real way to do it!

(Yes, and it seems like I've been flagged for saying it!)