Perhaps because the suggested standard enlarging lens for 6x7 is 110mm? Or such is the length I've always seen coming with 6x7 enlargers...

Most probably, however, 6x7 enlargers and accessories are being kept by people. Who owns one, knows something about image quality. 135 or 120 enlargers were much more common, IMHO, 6x7 has always been into the high-end range.