When I was a kid, my mother's Electrolux reminded me of a toy car, and when I played with my siblings, we often begged to pull out her Diamond Jubilee and wheel it around with the matchbox cars, the pull-back spring cars, the R/C cars, and whatever other wheeled contraptions we were playing with. Which probably accounts for some of its scratches and scuffs today
It had wheels, like a car, and the giant, round, chrome hose connection mirrored the giant, round, chrome VW emblems on my dad's cars. There were also other undeniable similarities between the DJ and my parents' VW Jetta. They were both similar colors (beige with black trim), they were both the same year, they both had chrome circular disc hubcaps, etc. I think this also accounts for why I'm not into other vacuums. One grandmother had Eurekas... they didn't look like cars, so I never cared. The other grandmother had Rainbows... they didn't look like cars, so I never cared. If my mom had owned a different canister brand or worse, an upright, I wouldn't have paid any attention to vacuums.
I was also intrigued by fans... ceiling fans, box fans, roof turbines, etc. So knowing that the vacuum, which was like a toy car, had a powerful fan inside of it only made me like it more.
Fast forward to today... if I could collect anything, it would be cars. I have two Volkswagens now, and I can barely afford to insure them both. If I had it my way, I'd own about five more. But they're expensive to buy, maintain, restore, insure, own, etc., so I only have two. But vacuums, like others have said, are easier to collect. They're cheap, easy to fix, and you can cram about 8 of them into a closet, which is where mine live most of the time.
It has nothing to do with keeping a clean house. I'm not a hoarder or dirty, but if you walked in, you'd swear I didn't have even one vacuum, much less eight. Cobwebs in corners, dust under couches, evidence of weeks between vacuuming... I don't have any fascination or romantic ideals about a clean house.
Part of my collecting now is what I call my "one of each" syndrome. Like a kid, I often want one of everything. I don't collect much, but if I had a car collection, I'd want one of every body style/major cosmetic change. Similarly, I would like one of every "square body" Electrolux that vaguely reminds me of my past... I have a 1205, a Silverado, my mom's Diamond Jubilee, and a blue body Ultralux (I'm missing a Golden J, Olympia One, Marquis, and a Grand Marquis/Legacy/LX); I also have a blue body Epic, an all-white Legacy, a new black Legacy, and a Guardian Ultra (I'm missing the all-gray Epic, the white/gray Epic, and the white/blue Epic/Legacy). There were too many of the 2100s, Diplomats, Classics, Regencies, etc. for me to really care about them. The Epic/Legacy line is much easier to attain. Will I ever complete my collection? Probably not. I don't have the space, and I wouldn't be able to hide my strange obsession if I owned any additional vacuums. I would like a Golden J though.
And I'm not so attached to these things that I can't part with them. I've got my white Legacy on Craigslist right now as a backup plan in case my tax return money doesn't arrive before the due date of my car insurance bill, because car collection > vacuum collection. Similarly, my 1205 has a motor problem and has got to go. The armature was replaced after I purchased the dud on Ebay, but after the fix, it smells like something's burning. There's no point in owning it if it doesn't work, and the 1205 loses some of its appeal if it winds up with a louder 1401/1521 motor, so I'm ready to ditch this 1205 to find another.
So yeah, I think it's about 30% fascination with machines/cars, 30% reconnecting with the past/childhood toys/obsessions, 30% "one of each" syndrome, and maybe 10% feeling a connection with mom--because it was her machine. I wasn't allowed to play with dad's real cars, but her vacuum was involved in several epic battles.