"All five of the nominees for Best Featured Actor in a Play have appeared in a musical on Broadway, and all of their five previous nominations were for musicals."
I've noticed that in recent years, the Featured Actor/Actress in a Play awards often go to actors who have a history of doing musical theatre. In the past 5 years alone, we have Bertie Carvel, Celia Keenan-Bolger, Nathan Lane, Annaleigh Ashford, and Jayne Houdyshell. Then others were nominated, including Megan Hilty, Andrea Martin, Saycon Sengbloh, Sarah Stiles, Bryan Tyree Henry, Nathan Lane again, and as you mentioned, the other 4 male nominees this year.
In my humble and un-researched opinion, this is symptomatic of the "musical bias" that very clearly exists in the Broadway community, and by extension, the Tonys. If you aren't a celebrity, then you'd better be a musical theatre person! (there are, of course, notable exceptions to this, but I do see it as a kind of trend in recent years)