I believe there are copious adverts in the US that have been running for many years with little change. Those would have been 4:3. Over here adverts have been required to be 16:9 since (I think) 2000? May not be such a requirement in the US. Plus of course doing modern commercials in 4:3 makes them stand out more, from looking at the video you linked to anyway.
The other difference is that in the US there was never really such a thing as 16:9 SD, so if an advert was made in SD then it will be 4:3, if it was made in HD it'll be 16:9.
There was I remember FOX and PBS being SD 16:9 in the early 2000s and I remember it also causing a lot of confusion.
I think it was probably down to local affiliates.