As banks branches are slowly closing and major banks are making the shift to online banking, they are having less and less time to focus on ATMs. Banks normally had to do ATM maintenance, refills, and more themselves, but as they are allocating more resources to more important stuff, ATM outsourcing is becoming popular.
Banks can only really maintain the ATMs near their building, but for ATMs in stores and not near banks, it was not viable to maintain those. Even now, more ATMs are often maintained by other companies as it is more efficient than having the employees and bank maintain it. It is not difficult to do, but because banks have more guidelines to follow compared to ATM companies, they have a lot of compliance rules they have to follow.
One recent rule that caused issues for banks was the update to Windows 10. It may not seem like a big issue, but it requires the software of all ATMs to be updated. Banks were also hit with EMV migration rules, so they have to focus on the more important stuff or their bank will be risking noncompliance issues.
These complications have made ATM outsourcing popular again as more independent companies are better prepared than banks. The outsourcing cost is still there but it makes up for the inefficiencies and compliance laws the bank needs to keep up to date with.