Since FSLogix app masking can also mask applications, directories as well as files and masking rules can be targeted towards group of users connecting to host pool virtual machines, so what could be the compelling reason that could encourage using MSIX app attach over app masking since both cater to similar requirement of keeping one master image with all installed apps but targeted towards different users.
They both work in different ways so it would depend on your use case, applications & application compatibility. FSLogix App Masking is a proven method of installing multiple applications into a single image and then hiding the applications to the users who have access to them.
MSIX App attach is still in public preview and won't becoming live until later this year. It will also require you to convert all your applications to MSIX format, and then the applications attach upon demand.
So if you need to do something now, use FSLogix App masking. In the future MSIX App Attach will be the better solution, as you don't need to install anything into the image (unlike FSLogix App Masking). But application combability will play a big part.