Reasons why I often don't use cycles lanes/paths:
The council build them but don't maintain them. So you get small stones, bits of trees, broken glass, etc on them over time. They really need to be swept every so often. Car tyres do a very good job of sweeping the roads clean so which do you choose - the nice smooth Tarmac road, or the gravelly, twig and glass infested cyclepath/lane?
Many of them are sub-standard and some are downright dangerous. They aren't wide enough, they cross from one side of the road to the other and back again for no apparent reason. They are often just converted pavements and often shared with pedestrians which means that you have to ride very slowly and give way to the pedestrians.
You lose priority over side roads and entrances. Again, this means that you have to keep slowing down or stopping, whereas on the road you retain priority over side roads.
So the council builds a sub standard farcility and we are expected to use it, whatever? No thanks, I'll take my chances on the road. I've been cycling for 45 years, done close on 200,000 miles on my bike and have only been knocked off once.