I think you can ignore your heart rate when doing strides because you're not at a steady effort level as well as the fact that there's always a slight lag in HR display. Everything is over before you know it.
I follow the P&D method of strides: Accelerate to max velocity over 70m then 'turn off the gas' and float back down for 30m. Walk back to recover, which according to some other experts, is important -so that your 8th stride is just as effective as the first.
Cadence increases with speed (but not linearly). The 180 figure is simply a guide based on the assumption that this is where muscle elasticity comes into play -so you spring back off the ground. Below this figure elasticity dissapaits into the ground and you have to rely more on musclular effort.
You can try this to feel the difference: spring up and down on two feet at 180 bpm, as if jumping rope. Then 'stick' at the bottom to reduce the cadence. you'll notice straight away that the spring has gone and you now have to actively push yourself into the air again. Return to 180 and feel the springiness return.
Practice by running in place at 180 using a metronome, then just fall forward. It will become more natural before too long.
One reason many struggle to keep it high is because they reach forward with their leg and overstride -which reduces your cadence and slows you down.