Yours is an interesting question, but it is like saying, "Why do some folks want a Harley-Davidson while others want a 'crotch rocket' Kawasaki Ninja?"
Two different things. Two different reasons for wanting each.
We have looked at a lot of Airstreams and thought about buying them, but we keep coming back to the same four problems:
1. We would have to get a massive tow vehicle to tow one. We've been down the road before, towing more trailer than our vehicle could handle. We'd need an F250 to tow a reasonable sized Airstream, safely (unless you are from Montreal, in which case a Dodge minivan is apparently acceptable, at least from what we see on I-95).
2. We would have to find a place to store it. We keep our Casita in a storage locker (no kidding, it fits right in, and I can push it by hand). The 'stream would have to sit outside, which means, over time, leaks. You know the drill. Floor rot, peeling clearcoat, etc.
3. It would not fit into as many places as we like to go. Backing even 27 feet of fragile aluminum is a PITA, and that first dent is a heart-breaker. How's that banana panel looking? Fiberglas is pretty dent-resistant, let me tell you.
4. Cost: A used Airstream IN GOOD CONDITION costs a lot of money. It is a lot of trailer, to be sure. But spending $20,000 or more recreational vehicle seems, to me, to be a lot of dough. I am looking forward to retirement, not endless debt. We paid $8000 for our 1996 Casita (used) and it is pretty much in like-new condition (even the carpet on the walls). Of course, storing it in the storage locker helps. If I had to sell it today, I would likely get the purchase price back.
Is it small? Youbetcha! But after living on a boat in the Keys, it seems pretty large to us (two guys and a greyhound dog). You get used to small spaces, living on a boat. And actually the bathroom on the Casita 17SD is larger than on some Airstreams. No, really. At least from the toilet perspective.
Last year, we spend two months in it, traveling to the Gaspesie peninsula (Quebec) Nova Scotia, Newfoundland, and Labrador. We are leaving shortly for New York, Vermont, and Montreal - again for two months.
Yes, it is a bit of a challenge sometimes, but then again, we can take it anywhere and not worry about it, and also fit it in to tight spaces. It is camping, not RVing, I guess. Albeit camping with your own bathroom and kitchen.
We've had four RVs, including a 27' fifth wheel, an 18' tow behind, and a 21' Class-C (which we drove to Acapulco, don't ask). We've had this one the longest and loved it the most of all.
It is built like a brick outhouse. You can't kill these things. In fact, it is a lot more durable than an Airstream, I hate to say, having looked at maybe 50 used Airstreams over the years (we keep dreaming, but those same four factors keep us from buying).
Would we get an Airstream? Well, we keep looking at them, and we keep buying lottery tickets. But I suspect that our interest in RVing will die off before we end up buying an Airstream. Having a tow vehicle in the driveway and an Airstream in a storage yard somewhere (in the hot Georgia sun) just does not appeal to me.
We did buy one - for my Brother-in-Law. A nice 22-footer from the mid-1980's. But he has had the usual problems with it - leaking in the back, floor rot, etc. And it seems he is never happy with it. Always tinkering, and now his wife wants a Class-B.
For a lot less than a class-B, you can buy a Casita, which has more room in it. Maybe THAT is the better comparison for a Casita, the Class-B Motorhome, not an Airstream.
Maybe smaller is better - you learn to live with what you have, rather than what you want.
We tow the Casita with a 2002 BMW X5. No, really. It is rated for 6500 lbs towing capacity (with a 5-speed manual, thank you) and the 3500lb Casita is hardly noticeable back there.
It makes for a very happy lash-up. About 17 mpg average, and no sway, no hassles, and easy driving.
Small has its advantages!
But still, I keep buying those lottery tickets. Maybe someday. Maybe not.
Here is my take on the Casita versus a Class-B:
From a budget perspective, tow-behinds beat motorhomes, hands down, IMHO: