I don't mind that it takes a little proof to convince regulatory boards to approve medicines for Phase I trials, but these days Phase I trials are fairly quick to get (assuming you can sell the doctors and patients on it).

My guess for the delay is production process and delivery mechanisms. There are delivery mechanisms where the combinatory effects lead to some serious toxicity.

Plus, how long do mice live for? I'm sure they are still taking down observations and collecting data. The article submission may be to help with funding.