720p24 isn’t part of the Blu-Ray spec, so it has to upscale it to 1080p24 otherwise all you’ll get is 720p60.
720p24 is part of the Blu-Ray spec, but is not part of the HDMI spec. Which is why you can have 720p24 on disc, but it’s never 720p24 by the time it reaches your set. It’s either 1080p24 (best), or something worse.