I agree with DSYoungEsq that you can make a cui bono argument for both Richard III and Henry VII, so that’s not decisive. But, one key point is that Richard had disposed of the princes’ claim by having them declared bastards by Act of Parliament, so it looks like he had dealt with the issue. By contrast, Henry VII marries the princes’ sister, Elizabeth, and has the Act of Parliament repealed, making her legitimate again - and incidentally making the princes legitimate again. If they are still alive, they have a much better claim than he does.
In Tey’s book, she makes two important points, comparing and contrasting Richard III and Henry VII, which she argues points to Henry VII.
First, while Richard III was king, there were various other claimants and possible heirs (his other brother George’s children, for example). He left them alive, going about their business.
Henry VII, on the other hand, treated these potential claimants differently: he either married the young girls off to good Lancastrians, or he had them executed on various charges, pretty much trumped up George’s son, Richard III’s illegimate son, and so on. Those that survived Henry VII’s reign tended to be finished off in Henry VIII’s reign. So, you have a pattern of Henry VII killing off other claimants to the throne, but no similar pattern for Richard III.
Second, and to my mind very significant, Henry VII never accused Richard III of killing the princes! If Richard had them killed, isn’t that just the plum piece of propaganda for Henry to sway the undecided? If you were Henry, fresh to the throne, wouldn’t you have set up a Royal Inquiry, to find out and publicise all the gory details, who Richard ordered to do it, where the poor little corpses where, and so on? Perfect ammunition against the monster. But, if the boys are still alive, and people know it, it just doesn’t wash. The accusation against Richard III is only made years later.