Antenna size.
2m antennas can be small enough to mount on a hand-held and still perform reasonably. 6m or VHF-low (30-50MHz) the antenna is fine on a mobile but just not practical on a handheld.
Hence, VHF hi police/commercial handhelds became widely used, VHF low never did, so in the 60's when hams started converting them and the 70's when the japanese started copying them, 2m was 100x more popular than 6m. The fact that most other countries did not have a 6m allocation meant the volumes weren't there for Japanese manufacturers either (same reason 220 is low usage).
Ham 450 band is right next to the commercial UHF band, which had some advantages in a big city, so those handhelds got converted too, and japanese handhelds soon followed.
I don't have any 6m gear, but its on the wish list, but talking to other club members, I'd say 6m is used more like an HF band and most activity is SSB or CW, It seems like that and 160m are the two difficult extremes of the HF bands and after all the serious DXer type hams get a 5BDXCC and get bored with 20m, they start playing with those two bands.
Poole