The vast majority of readers don't create art themselves and so aren't as artistically minded. As in, they won't analyze exactly why a thing looks good or how much craftsmanship is put into a black and white work. They just generally prefer color because, ooh it's in color.
Sure people still read manga, but as colored webtoons are appearing on manga sites at an increasingly rapid pace, people will flock to them and accept them as the new normal.
So Tapas, as they are trying to expand the company, is putting more of an emphasis on featuring work that makes them money, so mainly the premium titles, which most if not all are in color.