As a bloc they are also younger, so there's overlap with the underrepresentation of that bloc. Will take a few cycles to become more representative.
Surprised more people aren’t citing this as an answer. Irreligiosity at least in the US is generally a Millennial/Gen Z phenomenon. Likewise I contend irreligious underrepresentation is not the concern but rather Judeo-Christian overrepresentation. At the very least I want a Senate Majority Leader who doesn’t openly cite the Torah to justify Israeli settlements.
Related
Some might quibble with bracketing Islam and Christian Europe, or ancient Greece with Egypt and Persia, as part of the “West.” But it was the rise of Islam that fractured the world of Late Antiquity, united around the Mediterranean and stretching from Arabia to Scotland. North Africa and Syria produced Roman Emperors. Britain did not. “The West” in modern parlance is a secularized form of the archaic “Christendom,” but Christianity and Islam operate within the same metaphysical universe and partake of a shared history. Christians and Muslims both worship the same God revealed to Abraham. They have a truly shared understanding of what a religion is, though they might fight to the death over abstruse matters of theology.
Islam and Christianity both draw on Hebrew religious ideas and Greek philosophical traditions. Both have been strongly shaped by Persian Zoroastrian concepts of good and evil, heaven and hell. The angels who serve the Abrahamic God are imported from Zoroastrianism, as are the ideas of the bodily resurrection and the end of days. West of the Indus river, and north of the Sahara desert, the peoples of Europe, North Africa, and West Asia may have been fractious and seen themselves as enemies, but ultimately they spoke one cultural language.
In Islam there is a term meaning "People of the Book". This term's existence speaks to the truth that what you posted here reflects.
