They look cheap and vulgar
Yeah, 'cos that's what's really important when choosing footwear.
Crocs are cheap, light, comfy and don't get waterlogged... ideal for round camp, or in the garden.
For people who prefer style over substance I can see they probably wouldn't fit the "rugged" bushcraft image, but there's a reason Crocs are as popular as they are.
Plenty of reasons not to like them - they can get slippery if your feet sweat, they have holes which isn't ideal around thorns / thistles etc - but since when is "cheap" a bad thing?
My Frosts Clipper looks (and was!) cheap - less than £10 - but it's no better or worse than many custom knives 10 times the price.