I don’t think anyone can point to any particular reason “why she lost.” I think it was ALL the reasons, and it shows why we have to be on top of ALL of them.
Voter suppression, coherent message telling voters how their lives will be better, authenticity so they know you mean it, listening to and appealing to all components and regions of the traditional Dem coalition, outreach to new voters, keeping the other side from rigging things like voting machines and voter registration lists and illegal donations and activities and interference like Russia and Comey, and then delivering for your constituencies in order to help the constituencies AND the next candidate who runs. (Thanks, Obama.)
When you look at the record the Clinton campaign and Obama admin and Democratic party apparatus failed on all of those counts and still got more votes. Imagine the landslide if they’d actually been on top of all of that.