What attracted the Tyranids?

The Tyranids were first attracted to the Milky Way Galaxy when the xenos communication device known as the Pharos was overloaded in the Battle of Sotha during the Horus Heresy. It is possible that they have been preying on other galaxies since time immemorial.

Can the tyranids be stopped?

If the tyranids are unstoppable, the orks are able to stop them. Both of them have infinite numbers, adaptability and psychic strength on their sides. The orks are even able to make the impossible possible.

Are tyranids good in 8th edition?

Tyranids have a lot of good toughness, decent save, high wound models, as well as lots of cheap, average toughness, low save models – not unexpectedly. Some units such as my beloved Carnifex are pretty cheap for their survivability, whilst others such as Trygons seem relatively fragile for their points cost.

Are tyranids competitive?

Tyranids on face value probably sit in the upper mid-tier, especially after some minor buffs through the Blood of Baal. They aren't Space Marines, but they have a broad range of options that can all be competitive.