Probably because they weren't truly certain that Hillary was a shoo-in. Look how much trouble she had against old, socialist Bernie, and how small (and few) her campaign appearances were. Not to mention her health issues and general unlikeability.
To them it wasn’t risky. They believed themselves to be bulletproof. And Obama could have pardoned everyone. Perhaps they never envisioned how Trump could turn this around on them.
Even if Hillary won, it would only have occurred amidst massive voter fraud, which James O'Keefe videoed the Dems admitting to many times.
If Trump had lost, but contested the loss by spending millions to uncover the Dem deceit, they would then find very handy another way to discredit, and hopefully put an end to, Trump's political ambitions.