Because I suspect the police and the CPS know that an attempt to charge Andrew could be legally risky, and they will be mindful of their own reputation here - show trials that founder don't look good for them.
Mandelson’s position is much more clear cut - there is email evidence which on its face suggests he was clearly passing on secrets (as in, covered by the Official Secrets Act) and price-sensitive information. There are any number of criminal offences he could have committed here, from Misconduct in Public Office to market abuse. Lots of people are prosecuted for market abuse on far more circumstantial evidence than that.
Andrews’ position is tricker. Firstly, sexual offences are trickier generally (more he said - she said, though I doubt any jury would give Andrew the benefit of the doubt now). In relation to Virginia Giuffre, Sections 57-59 of the Sexual Offences Act 2003 introduced offences around trafficking into the UK for sexual exploitation. I bolded 2003 because the date is important here: that was after Virginia alleged Andrew and she had sex. So this matter would be looked at based on the law as it was before the Sexual Offences 2003 was enacted. Before 2003, the impact of trafficking on consent was not considered, and it is not retrospective.
Another point to note is that prior to 2003 there was a defence of a genuine though unreasonably mistaken belief as to the consent of the complainant - that has now been abolished. But that is relevant to assessing whether you could successfully prosecute Andrew for a sexual offence that occurred in 2001 with a 17 year old. I think the CPS and the Police would consider that to be a case that has a lot of uncertainty.
As for the new allegation about the trafficked Russian women who Epstein offered to introduce to Andrew and who allegedly spent the night with him after being shown around Buckingham Palace, without more it’s hard to know what the authorities think about their prospects there. Realistically I think the age of the woman would be a factor they take into account, though whether it would makes a difference to their decision is hard to say.
As for the rest of the emails shown to date, they show Andrew is a sleaze, and idiot and completely lacking in judgement (understatement). But unlike others I have seen, he is not giving Epstein permission to kill people, talking about torture videos or laughingly referring to Epstein’s liking for sex with very young women (Disclaimer: quite obviously I have not read all the emails! Who knows what else will come out!). They would play very badly with a jury about Andrew’s moral compass generally, but they do not in themselves reveal any criminal acts, I don’t think.
The trade envoy stuff may well lead to more clear-cut offences, and I think should now be investigated. Ironically the Bribery Act would probably be the easiest route to a prosecution, but Andrew was dumped from his role before that came into force. There is Misconduct in Public Office though - though it’s not a very clear offence to charge and prove.