They don’t want to get better.
Smurfs are a particularly difficult population to study, since most of them will try to conceal their nature. And even when cornered, their answers are to be taken with a grain of salt.
Yet, among those who can be brought to express themselves about it :
- A majority describe a feeling of unfairness/frustration at their original level of play. Dealing with asymmetrical balance can be frustrating at times, particularly in a fog of war game. When players become unable to cope with that frustration, they either stop or turn themselves towards smurfing.
- Stressful nature of the ladder. Playing normally eventually bring you at a 50% overall winrate, which you must play at your state of the art possibilities to maintain. Some players want to win in a relaxed manner, without having to really fight for it.
- Fun/trolling, trying nonsense strategies that would be doomed to fail against a fairly matched opponent.
- Others.
I must have the proportions of those, but didn’t publish them in the dedicated thread yet I think. If people manifest some interest, I might go back on that.
Stupidity can be a trait, since they indeed don’t realize that the more they degrade the ladder, the less likely it will be to attract new players. Bronze 3 is a caricature of that, since staying in that league too long in fact bring them to face more smurfs/challenge than regular players. Which what they were trying to avoid in the first place. 
However, they aren’t necessarily bullies. Some are, and those are often particularly toxic, but they are minority I think. Most are merely players unable to cope with one or several aspects of the game. 
Can be helpful. Particularly in the context of friendly coaching, where they review and explain your mistakes afterwards.
Sadly, most of the time that isn’t the case with smurfs. Their very smurfing is a hint they play only for their own sake ; and at another hand, most will deny even being a smurf, if they didn’t shut all communications in the first place.
Also, their is a mechanical requirement to correctly execute most strategies of that game. If you don’t meet them against a similarly incomplete opponent, that’s not a big deal. If you don’t, against someone who does, you will often die without even being able to set up your strategy.
Reapers pokes in early TvT are an example of that. I sometimes kill opponents of my level just on my 2->4 reaper poke, because my micro isn’t as bad as my mechanics. Would they have 500-1000 MMR less, they would probably always die on those reapers, and it’s not by seeing my micro that theirs would improve.
In fact, what’s probable is that they would rage, then rage even more by seeing the mismatch in my profile level, and don’t learn a thing.
Keep in mind that if smurfs can’t cope with the frustration induced by playing with players of their own level, it is unlikely for regular players to react better against opponents of a superior one. 
I’ve seen BeastyQT execute a magnificent and totally atypical mass nukes strategy. It was splendid to see how he could cripple an opponent by just harassing constantly.
And then I saw the opponent’s league, diamond 3, almost immediately concealed by changing tab. My esteem for BeastyQT’s stream decreased that day. Where’s the challenge in winning players that have half of your own mechanics ?
Ladder and unranked are performed in the same players pool. Team games also have smurfing issues, possibly more than 1v1 since they have botting as well.
Custom games is the way to go to play in a relaxed way without harming the game’s ranked/unranked function. But if you knew smurfs as well as I do, you’ll get that’s but wishful thinking. 
If a root solution is ever applied, it’ll have to come from the devs, which is unlikely.
We, regular players, have to deal with this by ourselves. 