A) Simply put, Clinton has represented herself as "Obama Term 3". If you don't like Obama, for whatever reason, you probably aren't voting for her. There are an non-insignificant number of Democrats, especially in certain states, who aren't super keen on Obama. It's as simple as that.
B) Regarding the bolded, I honestly don't think you can apply Occam's Razor and come to that conclusion at the same time. We know how states like West Virginia and Kentucky and etc. vote in the general and in past primaries. People aren't playing game theory here - they are voting Democrat because they have to (closed primary) or because there are local Democratic elections they care about. That doesn't mean they are Obama (and all he represents) fans.
I'm not super convinced. At least not yet - especially given a lot of the data we have out there (that for example Black Mamba touched upon). The ideology between Clinton and Sanders isn't that large. It's more about the methodology, the degree and the depth of policy. I feel confident that the vast majority of Democrats want higher wages, more and better healthcare, better and more affordable education, stronger protection for all racial/gender/sexual classes, etc. However there is more than one way to achieve those goals and some ways are easier/better given the make up of the country. I also don't think the left-most version or interpretation of a policy is always the best or most practical. We'll see how the next years play out. I just pray it remains civil and don't get a "Green Tea" movement started. I think what has happened on the Right has me shook.
I've never bought the "too lazy to change" rationale. The "there are other things on a primary ballot aside from presidential candidates" rationale makes complete sense to me though.
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