Abby D. from the Google Maps team explained that now when "you visit My Places on classic desktop Maps, the red “CREATE MAP” button will link to Maps Engine Lite (Beta). If you prefer to continue using classic My Maps, simply click on the “Or create with classic My Maps” link."
Here is a picture:
Abby added, "your list of My Places also now includes Maps Engine Lite maps that you’ve created, in addition to your My Maps and starred locations."
There have been no responses to the thread, so I assume most people don't care too much about this change.
Forum discussion at Google Maps Help.