I question the premise that he's still Disney's go-to guy for original musical scores. He's not done as many original scores in recent times as he had been in the early to mid 90s. He did about one musical per year with Disney starting in 1989 until 1997. Most of his recent work with Disney has been adapting his past film scores for the Broadway stage.
It makes complete sense for Disney to use Menken for stage adaptations of his previous scores because it maintains his voice in the project, particularly since he got started in stage musicals. Before he ever worked for Disney, he did the stage version (and later the film version) of Little Shop of Horrors.
Additionally, Disney has a history of using him to adapt his own scores for the stage... starting with the highly acclaimed 1994 Broadway production of Beauty and the Beast.
His Wiki page lists the following projects with Disney:
- Beauty and the Beast - 2017 (live action)
- The Hunchback of Notre Dame - 2014 (Broadway)
- Aladdin - 2014 (Broadway)
- Newsies - 2011/12 (Broadway)
- Tangled - 2010
- The Little Mermaid - 2008 (Broadway)
- Enchanted - 2007 (live action)
- Home on the Range - 2004
- Hercules - 1997
- The Hunchback of Notre Dame - 1996
- Pocahontas - 1995
- Beauty and the Beast - 1994
- Aladdin - 1992
- Newsies - 1992 (non-animated)
- Beauty and the Beast - 1991
- The Little Mermaid - 1989
Particularly starting in the late 90s, it doesn't seem like he's been working with Disney as much... here's a list of all the Disney musicals he didn't do1:
- Frozen - 2013
- The Princess and the Frog - 2009
- Brother Bear - 2003
- Tarzan - 1999
- Mulan - 1998
- The Lion King - 1994
In general, it seems the production of Disney musical films dropped significantly as we entered the 2000s and, despite him scoring every Disney Musical between 1989 and 1997 (except The Lion King), Disney did two films without him in a row, leaving a gap in their association between 1998 and 2004, and then he seemed to do about every other musical film between his recent work on stage adaptations, which started in 2008.
1 - I've left off a some films I feel highly music-centric but that probably don't qualify as "musicals" like Lilo and Stitch.