>>1926884because fun :)
not everything is about speed and efficiency thats the only disadvantage of smaller wheels
the advantages are numerous but mainly they just look fun, i like small wheels theyre nimble like a bmx and the bikes are much easier to store or transport
tyres rims and spokes are cheaper
makes the entire bike smaller and lighter
personally for an mtb id rather have 24 or 26 inch wheels but those dont exist outside kids bikes or fat ebikes anymore so 20 it is i guess
for an urban environment 20 is perfect
