There are two very different yet both equally harmful types of abuse being discussed in this thread. Sexual abuse (not relevant to Oppergard) almost always occurs in secret. In contrast, emotional and physical abuse very often occur in public. With sexual abuse there may be whispers and innuendo, but with emotional abuse it's frequently all there for many to see. So why was it tolerated? Well, there are a number of possible reasons.
First, most people are reluctant to insert themselves into other people's business. They might see something that they think is inappropriate, but they leave it to those directly involved to address it.* (I'll come back to this one.)
Second, and this is by no means an attempt to excuse the conditions that allowed this to foster, but pre Safe Sport, the US Figure Skating grievance policy had very specific constraints that made it very difficult for anyone other than the immediate victim to initiate a complaint, and it had to be done within 60 days of occurrence. This frequently led to complaints being dismissed because they didn't meet these highly-limiting criteria.
And third, Oppergard was a package deal. Disciplining him risked upsetting Karen Kwan, and by extension the Kwan skating dynasty. I have seen this happen in another coaching team situation in which one half is notoriously prone to lacerating rage. Addressing the abuser would risk losing the other half of the team, which can give pause if that duo has a large volume of business and is well-connected within US Figure Skating. It's not right, but I'm mentioning it because it can cloud one's judgment.
As for why those directly involved don't come forward immediately, there are myriad reasons. Skating is a judged sport, so there is always the fear that ruffling feathers will come back to bite you. But further, being the victim of emotional abuse can be very confusing and victims often look to themselves first to question whether they may have done something to cause the outburst. And if it happens a few times, victims often start to believe that they somehow are responsible for the behavior, which erodes self-esteem. The thought process is, "If I'm so horrible that I deserve this treatment, who else would want me?"
It's easy to make assumptions about how you would behave if you're not directly involved, but it's not as simple as it may seem. It's also very difficult to speak out, so I applaud those who are doing so now. It's never too late to seek justice.