Why I surmise is that Hillary, while a great and experienced candidate, will not change the status quo heavily. We won't regress, but significant change will not happen.
On the other hand, if Republicans are swept out of office en masse, significant change will happen (including, with a massively lopsided turnout, the potential end of the Republican party and creation of a new liberal party, but that's still unrealistic given that Republicans aren't dead yet). Much more change than would happen with a one-time effort of electing Sanders while letting Republicans maintain power. But that requires a lot of people to vote.
Sanders worries me with his emphasis on free college. A great goal, but not imo the absolute top priority (justice system reform, infrastructure, health care, pre-college education, cheaper college).
One of the most socialist policies in American history is also one of the most devastating policies in American history for Black people. Look up Ta-Nehisi Coates on the FHA. A socialist policy is not immune to racism or inefficiency, and we need to be very careful that it doesn't have such an effect. Free college as a top priority might greatly help people with money (who would not have gotten financial based aid) and people from strong pre-college education systems (thus getting into top schools). It might not be as efficient at helping the poor or those from school systems in dire conditions, who would benefit more from targeted initiatives.
If Sanders is too ideological to crunch the numbers, consider efficiency, and decide which to prioritize first, that is a concern.
But that's kind of beside the point, because the Republican party, at its core, is intellectually bankrupt. It's hard to make this statement without sounding like "X vs Y", but I honestly have no warm, fuzzy feelings for Democrats. It's just that a rational observer will look at the "trickle down" economics, the prioritizing of low taxes for the most wealthy, anti-science and racist rhetoric, the underhanded reduction of options for womens' health, the suppression of voters, and on and on, and determine that the current Republican party needs to go.
Furthermore, recognize that the Republican party has absolutely no qualms about ruling without majority support. They will not hesitate in the slightest to disingenuously claim a public mandate even if they knew most of the country hated them. They will not hesitate to increase inequality to the fullest extent they are able. Every bit of power they grasp will be used to hold the country back. If you are trying to lock them in a closet with a box of cigarettes until they get sick of it, you are deluded. They'll smoke 'em all and ask for more.
The solution is absolutely not to bet everything on a one-time effort for Sanders so you can put up a "mission accomplished" banner while Republicans don't let him do anything, and he potentially fails to prioritize the right thing if a glimmer of opportunity arises. The solution is harder than that. And it requires a whole lot of voting, not just one vote to end all votes.