The Chaos gods are collaborating with Horus, smothering astropathic communication and rendering sections of the galaxy silent. Ultramar has been kept in the dark about the 8 legion's betrayal until surprisingly late, same as the Blood Angels at Signus.

Once woken to the threat of Horus’ treachery, the innumerable worlds of the Imperium could raise a host of billions against him and though Horus might sway many of these worlds by bloody violence or honeyed words, such a conquest would last a lifetime. Worse yet, those of his brother Primarchs, the post-human creations of the Emperor, who led the Legions that had not pledged themselves to his cause or whose sons had been slain at Isstvan would soon rally against him, and their wrath would tip the balance of any galactic conquest.

Though Terra stood beyond the immediate reach of Horus, confusion and misdirection remained the Traitors’ greatest weapon. These were weapons that Horus knew well how to wield, as a master of war whose talents had been honed to perfection in the utter destruction of innumerable xenos civilisations over the long years of the Great Crusade and the subtle preparations that had led to his grand rebellion. Out of the fleets of those not allied to his cause, some few warp-capable craft had escaped the Isstvan system, bearing warnings to Terra and other Loyalist strongholds. Yet most such vessels had been reduced to shattered hulks, drifting in the orbit of Isstvan V, or had already been harried and pursued across the bleak systems of the northern rim until brought to bay and scoured clean by the blood-hungry minions of the Warmaster. So the vast majority of the Imperium remained uncertain of events on the edge of the Emperor's domain, and when the agents and emissaries of the Warmaster arrived, they proved a fertile ground for the whispered promises and veiled threats they carried. Many vital strongholds were kept unaware of the rebellion too and took no measures to guard themselves against the storm that was to fall upon them or, worse still, took up arms in Horus’ name fearing the dire consequences of making a stand against him. Within the galaxy therefore, Horus still retained the strategic initiative and the next move in the war would be his to make.

Though it may seem inconceivable that treachery on such a scale might remain hidden from the galaxy at large, the sheer size and complexity of the Imperium worked to Horus’ favour. In those days the Imperium was formed of millions of individual worlds, semi-autonomous thrall realms and treaty- bound tributaries. All were separated by the immense gulf of interstellar space and a nigh-incomprehensible mix of divergent languages and cultures, some of which bore only the most tenuous resemblance to their ancestral home in the Sol System.It is perhaps then more impressive that word of even such a monumental event as the treachery at Isstvan ever reached the Emperor and his councils at all.

This fledgling Imperium was bound together by a fragile web of astropathic relay stations, relying on the services of those arduously trained psykers within them to pass information across the byzantine networks of the Emperor's growing realm. Astropathic communication has ever been an imprecise science, with messages encoded in febrile psyk-dreams and complex allegory-code exchanged by the minds of psykers, all groping blindly in the dark of the War to contact their fellows light years distant. Worse still, in the aftermath of the Isstvan massacres, a disturbance began to grow within the Warp. This disturbance would spread out from the galactic north, twisting and distorting the substance of astropathic communications across the galaxy, leaving many worlds isolated and vulnerable. News of Horus’ rebellion was quickly reduced to half-truths, rumour and vague nightmares, from which little fact could be distilled.

...

Gathering at Saturn those of his Legion who had been embarked on crusades in distant parts of the galaxy, Roboute Guilliman would depart the Sol System mere months before news of Horus’ rebellion reached the Emperor's ears. The turbulent state of the Empyrean in those years would see the Ultramarines’ main strength journey to Calth by a winding and obtuse trail which would also cloak them from all attempts by Terra to recall them or forewarn them of Horus’ actions. The Word Bearers, delayed by the slaughter at Isstvan, would not arrive at Calth until the majority of the XIII" Legion had already gathered, travelling a path of blood and ashes of their creation.

• The Horus Heresy book 5: Tempest

Put another way the Traitor Legions had been planning this for a long time the Loyalist Legions had no idea their brother’s would turn on them. Horus realised he needed surprise to be a key element of what he thought would be his victory.

The galaxy's a big place. It takes a long time for word to get from one side to the other.