[Rems]

Remember, it was a mad dash for Terra once the Horus Heresy started.

Horus didn't have the time or forces to crack a Primarch's homeworld's defenses. There was no better time for Horus to attack Terra than the second the Drop Site Massacre finished. Every second wasted on taking out Medusa, Nocturne, or Deliverance was one second more for the Dark Angels and Ultramarines to come to the rescue. One second more for the Imperial Fists to fortify Terra.

Which begs the question of why it took 9 long years to get to Terra.

 

[Rems]

Even with the help of the Chaos gods, it still takes a long time to travel the galaxy. Plus a bit of refitting after the mauling of Istvaan III and V.

Not especially. From the Eastern Fringe to Terra is a 6-8 month journey (3rd ed rulebook).

Then clearing the path to Terra as well. Look at what the Alpha Legion had to do in Extermination.

Did they really need to clear a path to Terra though? It's not like they need to travel by land and their forts standing in the path of Horus' armies after all. He could simply jump in the warp and pop out in the Sol system.

There never would have been a better time to strike Terra then after Isstvaan. Terra was not yet fortified and it's only defenders were a substantially weakened Imperial Fists. All of loyalist legions were mauled, scattered across the galaxy or unaware there had been a betrayal, some all three.

Horus had the concentrated force of 7 legions. He could have crushed Terra.