The problem is that you are reading a jpeg files as your input. Jpg is lossy so your 'white' pixels are not fully white. You can fix this by setting a ColorFuzz percentage. You might have to tweak the percentage for your situation but your input file converts properly when I used 10%:
using (MagickImage image = new MagickImage(@"D:\image\input.jpg")) { image.ColorFuzz = new Percentage(10); image.Trim(); image.Write(@"D:\out.jpg"); }