Maybe it's their devs not having HDDs anymore. Who knows. I'm pretty sure it's not the volume of data being read/written. Just a lot of more smaller reads/writes or less attempts at latency-hiding.
Be it the anti-malware, less carefully written system apps or whatever. I think especially XAML stuff had terrible start-up times on HDDs. I remember filing a feedback item because after booting up my laptop it took more than 40s to open up calculator.