It's a combination of factors.
Online - lots of people have had issues with the website not accepting answers, not allowing you to move forward, crashing. People who don't "do" online have struggled to get a paper copy.
Year late - the Scot Gov cancelled doing the census to "keep us all safe" from Covid, show how much better and more caring they are than that nasty bunch of Tories in Westmonster. (Sorry Wales and NI, you don't get a look in, their rhetoric is all about Boris). Because 95% of the UK did their census in 2021, there is not as much buzz around it.
Advertising - they have put ads on telly, radio etc. But the way we watch telly and consume media has changed. Lots of under 30s won't have ever seen their ads on terrestrial TV.
Political questions - not just about trans history and sexual orientation, but also about how you define your nationality. Saying you identify as British and not Scottish is a HUGE political statement in Scotland. People fear being put on a SNP list to be reeducated into thinking properly.
Language - census bumf in English only. Probably available in Gaelic too, they love a bit of Gaelic. 🙄But if you speak Polish, Urdu, Punjabi, Spanish, Xhosa - you're on your own pal.
The whole thing is an utter disaster, has cost £££££ and is just another in a long line of SNP fuck ups.