Derek was referring
to this post
As to the reason, believe it or not, that's one of the main reasons. What happened is that some companies reacted quicker than others in a period in which there was a huge technological switch and that made things complicated. The first thing is of course about manufacturers that released TV that were able to see BT2020 only without the HLG color curve as I mentioned above. In that period for instance (I believe 2015) we rolled out UHD in an H.265 stream with BT2020 SDR 100 nits. That worked like a charm. Many companies did the same but when we had to release it across the group as well like in Italy and other countries two years later, there were HLG and PQ already and we wanted to have a process as consistent as possible by launching HDR but also keeping old users with the old UHD BT2020 SDR option to be able to see contents. Being like "you can't watch UHD anymore unless you buy a new TV from today" after as little as two years wasn't really ideal for users... So we ended up with the HLG compromise as we didn't really want to air two separate 4K streams and waste twice the bandwidth, especially 'cause it's extremely costly on HotBird as it's really overcrowded. Sure, people may say that internet based decoders will be the future, but not everyone has a good connection and you're also moving the cost from a satellite to a CDN which isn't exactly cheap (although it's way cheaper than the satellite), which is why we're inviting customers to subscribe to our fiber internet offer.
Anyway, I'm digressing.
The thing is that this is our situation and many companies did the same. You also mentioned the cameras and that's true, for live sporting events they can shoot directly in BT2020nc HLG 10bit which is a big added bonus 'cause you can easily connect them all via SDI, route them to your beloved video server, adjust them and you're fine. You can also live downscale and tonemap the signal to FULL HD Linear BT709 SDR 100 nits and since it's a 50p stream it can be easily divided in fields to get a 25i all live with specialized hardware so that it can be aired in FULL HD as well (and in 720x576 25i Linear BT601 SD from the FULL HD stream for what matters). So it's a win win. If you wanna go to PQ instead, it becomes a bit trickier. Cameras don't generally shoot in PQ, they either use Linear BT709, HLG or Log (where Log can be SLog, CLog, LogC, FLog, VLog etc) depending on the manufacturers to get as many stops/nits as possible. Of course you can't air in Log (although I've had much fun laughing at other companies that did this wrong and ended up with things like highlights in Log on air by mistake xD) so you have to apply a matrix of Linear Transformation to go from Log to PQ and that has to be manually written every single time, for every event and possibly in the same even, adjusted for every camera. So you still have everything connected through SDI which goes to your hardware which performs the linear transformation and delivers it to the video server where you can adjust everything and go on air. Now, what about the ones with a TV that ignores the PQ colour curve? They're gonna have to watch the FULL HD Linear BT709 SDR 100 nits 25i stream... Right, speaking of which, how are you going to produce it? The same way as before? Oh no, that would be lame, you shouldn't start from an already linearly transformed signal (the PQ one) and tonemap it to Linear BT709 'cause your original signal is in Log, so you should start from the Log. Therefore, you're gonna need a second tech ops, encoding and ingest department that is gonna do exactly the same as the 1st one but instead of linearly transforming to PQ, they linearly transform to Linear BT709 and divide the 50p in fields to get the 25i. This is a mess, you have redundant people doing the same thing and you're gonna have to put extra stress on your departments and have two separate production teams... And for what, since many TV and cameras were not able to go past 780 nits leaving alone 1000 nits 'till not so long ago? If you take all these things into account, then HLG suddenly looks like a feasible option...
Side note: I think that BT Sport actually tried a similar workflow but I'm not sure 'cause I don't work there but apparently some former Sky employees are working there and these were the rumors, go figure...
Side note II: this is what I learned from my very personal experience, things might be different in other countries and other companies, but from what I saw I'm pretty confident that we're all in the same shoes more or less.