My reasons are wide and varied.
I have (had) been a motorcyclist for the past 25 years .. starting on a scooter that was a few years older than me at the time, the latest being a heavyweight sports sidecar rig that i put together over a few years of garage time. I still have it, but it's hibernating in the garage now - a health crisis starting a few years back, going "house-poor" (slave to the bank on the 25 year plan) and a sort of mid-life crisis all going on all at once, saw my former motorcycling passion, along with the attendant related expenses like insurance, maintenance, gas, registration, etc etc put on the back-burner.
I have flitted with MA bicycles for a bit now, though. My first experience was with the incomplete remains of a Bianchi Aquilotto (http://www.mopedarmy.com/photos/brand/144/14724/
) in the early 1990's while in my mid-20's, that i had fished out of someone's garbage a few doors down. A few hours doing some McGyvering with a rigid MTB, saw me doing 60 km/h down the black-top with a big foolish grin on my face. I think i was hooked then..
I built my first purpose built MA bicycle about 5 years ago - therapy for a period of time in which my legs were failing due to need for hip replacements. I could ride a bicycle again, albeit the initial start up was a ***** to do, as a total gimp reliant upon canes otherwise!
Recently - even though i still have, and use that first Nishiki MA-MTB, I was getting drawn towards the relatively new Motopeds .. it was calling to me in the worst way. It took a few weeks to realize - I already have a few older versions of this in the fully motorized variety of a Honda CT70 and C70 .. both presently not being used! What is used, is the Nishiki - but i knew it could be improved upon.
So here we go again, back into the cycle, literally and figuratively.
At their base level, they appeal to me for the McGyver'ish tinkering opportunities - the chance to fling metal shavings around the garage making this part or that. The chance to mis-place a tool that i just had in my hand a few minutes ago. A chance to spend quality time thinking and doing something, that does not involve real-world pressures - relatively cheaply.
The actual riding, is almost an afterthought. I only have three or so tanks of gas in the Nishiki since putting it together - mostly in part due to those same health issues that made me seek out a build as 'therapy' in the first place. They are illegal to ride on the road in these parts (Ontario, Canada) - but there is a fairly large forested area right next to my home, that is beckoning me.. the Nishiki was/is limited in that department to handle off-road. The legs are getting stronger - opportunity, and a long winter, knocks!