Reason why bike scratched up in this location under crankset After owning my bike for 8 years and admittedly not always paying the most attention to it, I was thinking about buying a new bike soon. So I thought before buying the bike I'll see where damage is to my current bike so I can try and avoid damaging it the same way in the future. 

A lot of videos I've watched online advise you to protect the toptube, downtube, chainstay etc. Looking at my bike though I have a lot of damage under the crankset and was wondering if this is a common thing? Is there something I could have done to prevent it etc. It looks really badly scratched up and I'm not sure what caused it so any ideas about that?
My guess is that this was caused by chain suck. This is a problem where the chain doesn't roll off the bottom of the chainring as readily as it should, and the chainring draws it up toward the chainstay. It can get wedged between the chainring and chainstay, and take off paint in exactly this way.

This can be caused by worn cogs, a worn chain, a too-long chain, or too little tension in the rear derailleur holding the chain taut.