According to YouTube who defines what it takes to have a channel deleted, “A YouTube channel is terminated if it accrues three Community Guidelines strikes in 90 days, has a single case of severe abuse (such as predatory behavior), or is determined to be wholly dedicated to violating our guidelines (as is often the case with spam accounts). When a channel is terminated, all of its videos are removed. In Q3 2018, 50.2 million videos were removed due to a channel-level suspension.”
Breaking the numbers down even further, it seems that a good majority of those channels were deleted due to spam, misleading content, and scams. About 12.6% were removed for nudity or sex, 4.5% over child safety concerns, and some were also removed for harassment, promoting extremism, and also some for impersonation.
YouTube has come under fire in the past for taking too long to remove videos, or somehow allowing certain videos to be posted online. For example YouTube Kids was at some point found to be hosting some disturbing content such as conspiracy videos.