Cause the traveler always ran away. He also needed it to attack him so he could us it's energy to open the portal like we saw in lightfall. On earth it was only possible because Savathun used humans to perform a ritual that tethered it to the last city. Th ritual was broken in witch queen but by then the traveler was surrounded and couldn't flee so it's only option was attack, which was what the witness needed.

Anytime the Witness’s forces encroached on the Traveler it fled, it’s not just about connecting the Veil. It’s also about confining the Traveler, which is what it did in the opening cutscene of Lightfall.