Necrons reasons why in dot point form-ish-ness
- Currently developing a device that can separate the Warp and the Materium.
- If they take mars and release the Void Dragon the Mechanicum will most likely worship the C'tan and the 'Machine Spirit" will take over most of the Imperiums technology and turning it on the Imperium.
- Already have a device that can do what point one does but only on a planet
- The Deceiver spent years getting rid of anything that could stop the Necrons harvest
- The only thing that could stop them now is a rotting corpse that won't wake until Chaos is gone. Which is the only reason they don't hurry there research into the device explained in point one.
- Have already landed on Mars with 5 of there most basic craft. Above where the Void Dragon is imprisoned which means that they know where exactly he is.
There are many more reasons why.
Tyranids reasons:
- They're endless.. It took two of the greatest chapters to take on just ONE hive fleet. There are probably thousands more hive fleets headed towards the Milky Way
Thats about it.

Alot of people say these two will conflict eachother... Not really. The Necrons have time on there side. They're immortal. The tyranids don't go for tombworlds or planets with Necrons on them due to a psychic field or something produced by the Necrons that repel the Tyranids...
The only thing that could stop them now is if some miracle.. Or chaos ceases to exist for just one millisecond. As this would make it possible for the Emperor to heal himself. Which would spark a massive moral boost in the Imperium and probably cause a massive surge of expansion and zeal. Making it a hell of a lot more harder for the Necrons to take a foot hold in the Galaxy.