All part of the Emps. plan.
Yes, they did take longer to get there - but the Emp. knew they were coming. Why go up to meet Horus when it was just a matter of time before the other 3 Legions showed up? Horus knew it was hopeless and the Emp. had won, why go to his ship?
This is just how the Emp. rolls. I can not believe he didn't think that Lorgar would turn after his Humiliation, nor that Magnus would stop using sorcery. I think he ultimately planned the Heresy.
There are inconsistancies like this throughout the whole HH series. Here is an easy one that would have stopped the whole Heresy. Why did the Luna wolves put Horus in a stasis field and than take him down to Davin? All they had to do was trek back to Terra and deliver the fallen Warmaster to the Emperor and the Emp would have cured him (I think that would be well within his power, he did defeat the full power of all 4 chaos gods at once). The stasis field would have worked, it has kept Papa smurf 'alive' all these years.
We are talking about a being that defeated the Void Dragon as part of a plan that streteched over 45k years. How in the heck could he not have seen all of this comiing?
Pappa smurf and his entourage didn't show up in time because the Emperor did not want them to. They may have thought they had free will, but the Emp. was the puppet master all along.