I believe this is due to several factors, perhaps the most prominent of which are privacy-related issues and high fees, which have led to insufficient demand for these devices among users, as many prefer to use online trading platforms. Privacy doesn't just mean completing secure transactions and protecting personal data from any suspicious use; it also entails the potential legislative restrictions that could enable authorities to monitor citizens' financial activities. This has led many to avoid using Bitcoin ATMs, even if they are available to them.
Another reason I find important is the rapid technological development, which means a large number of ATMs quickly find themselves outdated and in need of structural updates to keep up. Consequently, operators of these types of ATMs will find that operating costs are disproportionate to market demand, making them a poor investment.
I agree that not many people are using Bitcoin ATMs because of several important reasons. Main problems are privacy issues and high fees which have made people not want to use them. Instead of this they prefer to use online trading platforms. Privacy means not only keeping transactions safe and personal information protected but also being worried about laws that could let authorities watch what people are doing with their money. Another important reason which you already mentioned is that technology is moving forward very quickly which makes many ATMs outdated and in need of updates to stay useful. So people who run these ATMs have to pay a lot of money to keep them running which is not good investment because not many people are using them.