They have been investigating Trump and his ties to Russia at least since the summer. They just chose to keep that bit of information secret. Comey didn't choose to do that for Hillary.
Hillary was treated utterly shabbily - unlike any other under investigation, and frankly I think we deserve answers on that.
When Trump was under investigation - NOTHING was said about it, and when it was over - all that was said was nothing was found wrong. Essentially - that was exactly what should have been said about Hillary - nothing wrong, no charges - but Comey didn't - he singled her out for a particular public flagilation - again - UNLIKE ANY OTHER. Given it's closeness to the election, and last minute surprise it is impossible to think there isn't something crooked there or at least unethical.