Jan R. Luth Why do we sing at a lower pitch?
Het ORGEL 97 (2001), nr. 6, 22-24 [summary]

Many Dutch church musicians experience that congregations have trouble singing ‘high’ tones like d2. Is this a new development? And if so, how can we explain it?

Writings of Van Blankenburg (1745), Kist (1840) and Van der Dussen (1848) indicate that congregations formerly sang both higher and lower than they do today. Chorale books in the 18th and 19th centuries have the hymns and psalms notated at the same pitch as, or at a higher one than, comparable present-day books. Important sources are: Witvogel (1730), Stechwey (1770), Michelet (1771), Potholt (1777), Van Eem (1780), Ruppe (1806), Hauff (1837), Bastiaans (1852), Worp (1892).

Organs often had a different pitch from a1= 440 Hz: Grote Kerk Alkmaar (415 Hz), Westerkerk Amsterdam (460), Oude Kerk Amsterdam (465), Pieterskerk Leiden, Stevenskerk Nijmegen, St.-Maartenskerk Zaltbommel (all three 415), Bovenkerk Kampen (g#1 = 435 Hz). But organists who played high-pitched organs did not always choose melodies notated at a lower pitch.

So indeed we sing lower than our ancestors did. An explanation might be the lack of singing education and of the urge to sing at all. In the 19th century, an average student sang as much in one day as an adult churchgoer now does in a month.