She's been hated since Bill was president. She wasn't a traditional first lady and they thought she was overstepping in terms of the administration and should have just stayed in her place.It goes back decades and it was a constant coordinated campaign from wealthy Republican operatives - Vince Foster bullshyt and all the rest. They saw her as a threat in the 90s when Bill was in power and kept going. When she later became senator, secretary of state and then a presidential candidate it went into overdrive. Not saying she wasn't sub optimal in some ways (remember Obama beat her in 08) but she would have at worst been a competent President even with big R opposition. But the cac backlash against Obama also was so intense it set the table for her loss.
She got close, but honestly she was always going to end up losing because their hatred for her. Hillary would have had to counterbalance by having Barack like charisma to increase voter turnout or have an exceptional campaign.