I'm sorry to hear what's happened. A number of things could have happened here, it could have been a half-arsed/biased/shoddy investigation, it could be that no-one else in the team wanted to speak up at all to substantiate the complaints, and so it was your word against hers (and since she presumably will have been able to speak to the investigator in person, whereas you didn't as you have now left, she may have been able to make a persuasive case to make them believe her). It could even be that the other people in the team might actually have said when questioned that the manager is lovely and there are no problems - I've seen the latter happen before, whether or not people believe it to be true, often they don't want to cause any trouble for themselves by 'accusing' the bullying manager for fear of things getting worse, or even feel they'll earn brownie points by saying how rosy everything is and this might make things improve?
Also, if the manager is seen as being 'high performing' or difficult to replace, consciously or sub-consciously senior managers or HR can be really reluctant to take action against them. It is a difficult situation to be in because if the manager has been there 2 years plus and has never had a complaint made before, you can't really legally sack them on the basis of one proven bullying incident, unless it was a really terrible gross misconduct type thing. You really need to give them a warning and a chance to improve with support etc., and to do this they have to go back to managing the team that they know have complained about them, leading to more drama, stress, bad feelings all round and often then a drop in performance, and then the situation gets really hard to manage. Not saying this is right or an excuse at all, just to try and explain the mindset that leads to a blind eye being turned, and the victims being painted as liars or over-sensitive or whatever...
As you've now left the job I would try my best to put it all behind you, I know that's hard. You know the truth of what has happened and although you didn't get a good outcome from your complaint, you should still be proud that you had the courage to speak up. And at least the next time someone complains (and there usually will be a next time), there is the start of an evidence trail/smoking gun that may lead to action being taken or at least the manager not so easily believed, so in the end it may have had some effect!