The maintenance part is one element but there is just something simple about SS bikes, no ‘right’ gear to be in, just pedal and you do the rest!
As you are in the same gear all the time you know when to pedal hard and when to rest, use your fitness and power to get up hills, pedal quicker at the bottom and keep the same speed all the way up.
Advantages over a geared bike is simpilicy and weight I guess. I had always ridden ‘geared’ bikes until I made a road bike with a fixed gear which I loved so decided to build a ss mtb and love it.
Very easy to convert, you’ll need a SS chain tensioner, a ss cassette conversion kit (spacers and a single speed cog) and a single chain ring up front. If buying second hand will cost about £20 – £30 quid, If buying new £30 – £50 depending on where you buy from.