Balancing keeping warm vs breathability can be tricky when it comes to winter kit. Although it can be cold outside, you can soon warm up on and bike and if your bib tights aren't breathable, you're quickly going to work up a sweat that will make you very damp on the inside.
The best bib tights will use strategically placed panels of different fabric to take account of where you need rain and spray protection, wind proofing, and where to provide exhaust ports. In general the front of legs get the most weather protection, while the back is more breathable, as is the upper body where mesh is often used to help heat escape.