She never got good ratings for two reasons:
1) Her time slot was also awkward -- She followed Chuck Todd and the last NBC News branded "straight news" program, and preceded Chris Matthews and all that is him. Her ability to be the merging point for those two just never materialized.
2) You also have to remember, some of the big breaking stories that have happened over the past six months either happened or was dropped by the print media between 4-6pm... and she just doesn't have the chops for breaking news. If you need an example, just remember Comey's firing day -- she had Dershowitz and some other person on, and they just weren't inspiring. I remember watching for maybe 10 minutes before dumping (reluctantly) to CNN.
I don't think this could be said enough. Outside of her weakness as a broadcaster, she has never had a good production value or (outside of the politicians) no good guests on any of her shows. Evem at FOX News, where your really don't need a good production team since the viewers are pretty damn loyal, her show seemed lackluster at best.
Even in the deep forest that is weekend primetime, I think she would have suffered. She's a lawyer by trade, and she'd probably have been better off as either a legal contributor to Rachel (since they're friends and all) or even as a Legal Analyst alongside Ari Melber, the networks chief legal correspondent (who ironically is taking her slot).
It's neither all her fault nor all MSNBC. They both fucked up.