The answer is that the Tory party was desperate for ground troops to fight Labour, and with a small and often elderly membership, this was hard to come by. It seems that their desperation stopped them asking the sorts of questions that an organisation with the luxury of many footsoldiers should have asked. They’d risk taking on someone like Mark Clarke because they considered it less of a risk to being utterly swamped by Labour activists in key seats.
But it’s not just Mark Clarke, who denies bullying Johnson. Those involved in Tory youth politics say bullying was rife – and not limited to one man. Perhaps the party judged what was going on to be the sort of usual histrionics amongst student politicians, who believed they were acting like grown politicians with verbal thuggery and internet smears.