Because they don't have all their needs met.
Because advertising constantly reminds them of what they can't afford.
Because the rich are too rich!!
Because they (the poor) no longer farm.
Because they are no longer needed for manufacturing.
Because they're packed into cities.
Because welfare breeds dependence and thus resentment.
N/A - the poor have always been pretty equally miserable.
"With me everything turns into mathematics."
"It is not enough to have a good mind. The main thing is to use it well."
"It is truth very certain that, when it is not in one's power to determine what is true, we ought to follow what is more probable." -- Rene Descartes
The easiest answer to the op is that they are not. The material conditions notwithstanding, the only reason why it seems to be the case that they are more miserable is that everyone has a voice that is preserved on the internet. If people could have posted online with free or cheap smart phones in 1912 you would see quite a great bit of misery that there wasn't a reporter around to write.
Now that isn't to say that what people feel now isn't valid, it is, but the past wasn't a place where everything was always better or worse for that matter but people remembered things one way or the other usually.
We are poor but not miserable. And I would challenge that we have all we need from handouts. Being on TANF my famil makes $513 a month plus $600 in foodstamps and medical. We dont use the childcare or take anything we don't need. We make $513 a month for working $35 hours a week on top of also filling out forms proving we are looking for work from various employers within our fields or below. Since the cheapest place we can get with utilities paid is $779 a month we sometimes fly a sign for work or help making rent and when we don't make it we live in the car then move when we have the money. Sometimes a hotel for the night or our old month to month apartment for a week. If you think it's just a matter of applying yourself youre wrong. I've already had a company owner on this forum tell me they purposley don't hire single mothers or mothers of young children and I've been fired from so many jobs because of my ADHD and PTSD that it's hard for me to find steady employment even though I've been to college and have a pharmacy technician license. My husband hasn't graduated high school yet but getting his GED is a part of receiving TANF so hopefully he will pass the English and we can at least be working poor again. Although almost all hope of earning something other than $8-$10 an hour has vanished. Every job we apply to now is commission based only and we cant make money worth a damn doing anything like sales. Especially in Vegas. If the government didnt help us our kids would be starving to death right now and everyone is all too quick to call his lifestyle entitled and lazy. I "work" 16 hours a day at finding odd jobs like helping people move or cleaning a house for a few bucks then 35 hours a week as mandated by TANF, am a full-time mother of three, and still have to find time to fill out the 20 pages of job applications required for TANF as well each week. People "collecting" a check actually have requirements to do so. You can't sit on your ass and collect welfare unless you are mentally ill o disabled.
I voted "other", and I think that this may be a question of being in debt. It is not a comfortable feeling if others are entitled to your future time as in contrast to your current earnings. So, the today's poor is rather like a slave, whilst yesterday's poor was more like a serf. I think slaves were always more miserable than poor but free men.