It’s because HDMI has a history of being a very unreliable connector that also introduces some latency into the system. (This is the short answer)
SDI became popular for major productions because its a secure two conductor wire that can handle runs up too about 250ft without issue, that requires no handshakes, nothing from the upstream side other then a feed to be received by the monitor downstream.
Most broadcast cameras are in a live on-demand environment where the ability to repair them or fix random cable issues is not possible so yes they will want dual feeds if they can get it. Most live productions also have a SDI return piped to the camera so they can command functions such as color correction on the camera via the booth. (ie Blackmagic style cameras and switchers)
So Yes, any professional production tech is going to want SDI as their main source since its reliability is just higher and not a habit.
Yes, having twin feeds out one HDMI and the other SDI could work but you start introducing issues such as added latency, how far can that run be ran before boosting, long term wear and tear not the hdmi port etc.
The hdmi port was just never designed to be used on a set, being pulled and un plugged, long cable runs etc… It’s not a bad protocol but not for on set use.