It's become less common to play in 4:3 over the years. The additional peripheral vision has become too obvious a benefit to not make use of.

It's probably mostly a habit thing. Most of the pros started to play these games back in the 90s, when 4:3 was the standard. Add in that playing all the low resolution options are 4:3 (and the 16:9 equivalents will add resolutions rather than take it away; the 1080p is usually 16:9, so 1920x1080, but 4:3 1080p does exist, and it's 1440x1080, which is a lower resolution), it's no wonder 4:3 stuck around as long as it did.