When the Tatiro Legions rebelled against the Emperor the far-flung Imperium was already in a state of flux as it tried to assimilate the lost knowledge and vast resources recaptured in the Great Crusade. THe forces of the Adeptus Astartes were in the process of re-equipping their forces with the masses of newly forged armour and weaponry pouring out of the workshops of the Adeptus Mechanicus when Horus unleashed his Legions.

As a result, the forces battling in the Horus Heresy fought with a mixture of the sometimes archaic armaments which they had used during the Crusade and previously unknown weapons based on arcane ancient technologies. Because the Traitor Legions were the ones that had advanced furthest from Terra they wre among the last to receive new weapons and armour. This meant that the Space Marines fighting for Horus lacked many of the weapons which would be subsequently phased into the arsenals of the Imperium over the next millennia.

After Horus fell the Traitor Legions were driven into the Eye of Terror, untouched by the flow of time. Their weaponry and armour has been embellished and decorated with personalised schemes but otherwise it has stayed unchanged down the centuries as they fight the Long War. Innovation and invention have become anathema to Chaos Space Marines as they battle the hated Imperium. They preserve their bitter anger for the lost millennia forced on them by the Emperor and anything new serves only to drive the shards of their hatred deeper into their tortured minds.

...

When the Legions began re-equipping, a number of weapons which would come to be in common use later were still experimental or even purely theoretical. Many other weapons relied on sciences that were barely understood but which had been found in ancient vaults of dark technology and copied for general use. For example, the Traitor Legions had no access to Melta bombs or multi-meltas, but melta-guns were commonly available. Likewise, portable (though dangerous) plasma guns and even pistols existed but the heavy plasma gun could only be mounted on a Draednought or a vehicle.
...

Autocannon were favoured as simple but deadly weapons that could easily be easily carried by a Space Marine in power armour.


p. 71, 2nd edition Chaos Codex

The Traitor Terminators, lacking the technology and equipment to maintain the notoriously fickle Assault cannon, still make use of the Reaper with its simpler mechanism and solid ammunition.

p. 73, 2nd edition Chaos Codex

Designer's Notes

Another interesting spin we came up with was to make the Chaos Space Marine forces feel as if they had been exiled to the Eye of Terror ten millennia before the Imperium's 'present' day. Though the technological advancement of the Imperium moves at the speed of an aged tortoise ten thousand years was bound to bring some changes. This suggested limiting the Traitor Legions' weaponry to some of the 'older' types and making others dangerous, experimental precurors of the guns used by Imperial Space Marines. This, combined wit hthe older patterns of power armour the designers have used, gives the Chaos Space Marine miniatures a dark, archaic feel which contrasts wel wit hthe clean, upright loyalist Space Marines.

...

Andy Chambers

p. 112, 2nd edition Chaos Codex