I maintain the DigitalOcean one-click images and did some testing with a LEMP stack for the WordPress image a couple years ago.
Originally the WordPress one-click was built using a LAMP stack because LAMP was at the time much more widely used and supported by the wider open source community. Most of WordPress’ documentation also assumes you are using Apache and some tweaking is required for WordPress rewrites on Nginx that is not required on Apache.
When I performed my review we did so to see if switching to LEMP would reduce base memory use on 512MB droplets (not a plan we offer any longer). MySQL can be a memory hog so that was our primary concern. My testing of the two alternatives found little difference in the base memory use.
The choice to stick with LAMP was made both for those technical reasons but also to ensure that a user spinning up a droplet with the stack would be less likely to encounter confusing information if they run into problems or want to customize their installation.
Our one-clicks are generally intended to be a quick-start to get going with a particular software stack but they are also often the first experience a person may have with the software. Taking that in mind we try to ensure that the one-click experience encourages that learning and provides something that as much of the community documentation outside DO as well as in our library will be applicable for.