Because he was ordered to.
There was no reason for this 26 year old to move to London and volunteer for an unpaid low level spot in a group advising the Trump campaign. There was no plausible reason for him to meet Australian Clinton foundation operative Downer in a bar in London and talk about Russians having files on Clinton that he could not possibly have seen.
I read on here that he was enticed with a plane ticket and a $3000 payment.