^
Powdered graphite (extremely fine) was considered the best lubricant for cylinder plugs (only) (the part that the key actually goes into) for many, many years. And it works really well......

...Until any kind of liquid, dirt, gunk, particulate gets in there. Bike locks get A LOT of foreign material into them. On interior door locks, where you won't see that happen, it works really well.
Once some foreign material gets into the cylinder, the graphite attaches to it and clumps up. It happens all the time when some 'genious' maintenance man has a lock issue and thinks that WD40 is a magic cure all. That makes everything even worse and more sticky when it dries. This negates its effectiveness entirely.

Don't use graphite on your bike lock.