I think that it's much better for marketing it to a newer generation. I suspect that for somebody in their early 20s starting to watch Seinfield these days a 4:3 ratio would immediately make the show look dated. I mean surely they ran the numbers and figured out it was worth it? Otherwise it wouldn't be so common with so many TV shows.
When I was a kid in the 90's I remember that it was not uncommon to have black-and-white movie recolored to make them look more contemporary. This is arguably an even bigger betrayal of the original artistic vision and I expect done for very much the same reasons.
The show is dated. Nobody has a mobile phone. Very few computers (I think Jerry has a Mac Classic in his living room). The cars and street scenes are all from the 1990s.
I'm not voicing an opinion, I'm just trying to explain the trend, personally I also prefer the original version. Clearly for the right holders and broadcasters the cropping is perfectly justifiable and actually preferred.
When I was a kid in the 90's I remember that it was not uncommon to have black-and-white movie recolored to make them look more contemporary. This is arguably an even bigger betrayal of the original artistic vision and I expect done for very much the same reasons.