Based on what devs have said, Enderal and Nehrim were given greenlight for standalone apps back in the days when Bethesda had different people in charge who made these type of calls. Now the chain of command got extended to Microsoft and people who made these calls in the past are no longer there
That being said, Fallout London will be released on GOG (not as an exclusive, they just offered to act as a hosting page for file download, it's apparently 40 GB package), so who knows, there is a chance it may get a separate install thanks to their help as GOG team have easier contact to Bethesda