"Credited" is the right way to frame this question, because it involves historiography as much as history.

This is going to be a long and kind of meta answer. I typed it out once, thought it might be too academic and political at the same time, and waited until I'd heard back from three colleagues (including an internationally-respected expert on evangelical history at the end of the nineteenth century). I'll try to be as concise as possible.

For a long time, histories of evangelicalism were "confessional." This means they were written by evangelicals for evangelicals, influenced by pre-1970 critical paradigms in church history. I'm not saying this older scholarship was un-rigorous or hopelessly partisan. Rather, it was produced by specific church historians for specific academic and non-academic audiences who held certain expectation. One such expectation was that these histories emphasized historical continuity within a church tradition. Why was continuity privileged over change? Because church history had one metaphorical foot in theology (which advances normative claims about what a religious tradition should be) and history (which advances descriptive claims about what happened over time). Of course, this distinction is itself a relatively new idea-- emerging in the late nineteenth century/ early twentieth century and unevenly "trickling down" through the different branches of historical inquiry. And, of course, critical theory and philosophers like Foucault pointed out that the act of creating historical narratives often advances "tacit" or hidden normative claims. I'm not talking about such tacit or implicit claims. I'm talking about professional norms shared by historians and their audiences. And, for a long, long time, historians of evangelicalism were expected to comment on what evangelicalism is or what it should be.

In the 1970s-1990s, histories of evangelicalism started entering the fields of religious history and intellectual history. At this time, evangelicalism was not considered "mainstream Christianity" by many religious historians. Scholars like George Marsden, David Bebbington, and Mark Noll, the foundational figures of late 20C evangelical history, were therefore advancing tacit claims about evangelicalism's historical provenance to other historians. They wanted to show that evangelicalism was a rich, deep tradition with its own intellectual lineage and coherent cultural theories. So, they tried very hard to recover "evangelical" thinkers like Jonathan Edwards and George Whitefield, and show how these figures' ideas persisted over time. At the same time, all three of these figures were themselves convinced evangelicals involved in normative discussions within evangelical nonacademic circles. Their histories were not blatantly confessional, but their idea of what counted as "evangelical" and "religion" once again privileged continuity over change.

Here's a quick example: David Bebbington, who is Scottish and not American, offered an in-depth study of the trans-Atlantic revivals with a special focus on Great Britain. He pointed out that even in the 1820s, "evangelicalism" was extremely theologically pluralized. And, unlike other Christian traditions (Catholicism, mainstream Protestantism), evangelicalism lacks a clear polity (leadership structure). So, he asked: Who counts as evangelical? What holds the movement together? And who wields authority (and on what basis)? To answer the first question, he created the Bebbington quadrilateral, a rubric of four theological emphases. These are bibliocentrism, crucicentrism, conversionism, and activism. According to Bebbington and scholars who use this quadrilateral, any Christian with these four theological commitments counts as evangelical. This allowed Bebbington to make sense of why Baptists, Presbyterians, Anglicans, and Methodists were all becoming "evangelical" while still operating within mainstream Protestant denominations. And the Bebbington quadrilateral was immediately adopted as a normative definition by the National Association of Evangelicals in 1989. Quick reminder, the 1980s was the moment when American evangelicalism was re-constituting itself as a politically significant national movement. The NAE even made common cause with conservative Catholics and marginalized inter-evangelical discussions of millennialism. Both of these moves would have been simply unthinkable for previous generations of "evangelicals." But, they were made possible in part by the Bebbington quadrilateral's redefinition of "evangelicalism" as a theologically unspecific Protestantism.

In the last ten years, the field of evangelical history has shifted again. Part of this was driven by the 2016 election, which highlighted the extent to which traditional evangelical histories had ignored or downplayed race. But the shift started around 2012 and 2013, because more scholars were studying "evangelicals" and recovering archives of correspondence, ephemera (tracts etc), and Bible distribution. These scholars, like Daniel Silliman, Paul Gutjahr, Candy Gunther Brown, Claudia Stokes, and Jan Stievermann, focused on the print cultures that helped "evangelicalism" hang together-- across the Atlantic Ocean-- and spread. At the same time, religious studies started paying more and more attention to how mainstream Protestantism coalesced as institutional and ecumenical religions in the early twentieth century. And, the success-- and crisis-- of American evangelicalism has created funding opportunities and policy needs for more histories of evangelicalism. A lot of this recent scholarship is ALSO aimed at academic historians, who care about granular descriptive accounts, and at "popular audiences," who care about normative claims about what evangelicalism is and should be. Benjamin Rolsky, a 20th-century historian of politically progressive Protestantism, has complained that a lot of recent popular histories of evangelicalism are just infighting between the conservative and progressive wings of contemporary evangelicalism. I think this is a bit of an overstatement, but there's definitely truth in it.

I agree that many popular historical accounts of American evangelicalism "credit" 19th-century evangelicalism with benevolence and social justice. But I think they're "crediting" Protestant social reformers with being evangelical (in a contemporary sense) more than anything else. That is, I think it's deeply ahistorical to say that the education reformers, abolitionists, and temperance advocates of the nineteenth century were "evangelicals" in the contemporary sense. But it's an easy mistake to make! Many of these reformers, like Harriet Beecher Stowe and Sophia Peabody and Henry Ward Beecher and so on, described themselves as "evangelicals." They, themselves, were focused on building pan-Protestant coalitions for political purposes! And, in the nineteenth century, "evangelical" was heavily inflected with Christian primitivism-- the idea that true Christianity existed back with Jesus and his disciples and [insert denomination here] were the true evangelicals. And revived, emotion-centric "evangelicalism" was not respectable for a long time but deeply wanted to become so. Indeed, as late as the 1870s, all sorts of marginalized Christianities are calling themselves "evangelical," arguing that they truly understand the Bible, and advancing claims to Christian primitivism! For example: the "so far politically left they break the scale" Universalist Unitarians call themselves evangelical until the 1890s, because the Gospels don't literally argue for the Trinity. (These guys are the BANE of the "orthodox Protestants" and later fundamentalists). Joseph Smith and his Church of the Latter Day Saints of Jesus Christ call themselves the true evangelicals, because they have the truest, most up-to-date "translation" of the Bible! Mary Baker Eddy and her Christian Scientists also claim the term for themselves. And so on and so on...

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