Pentecost, for Christians, marks the birth of "the Church." For evangelicals, however, church history generally means when their local congregation was founded. As a body of believers with no history, by design, historical holidays (I.e. Anything not explicitly about Jesus) fall to the wayside.
From an historic viewpoint, the Church Universal is an amazing institution. Like all institutions, however, tradition tends to shape it, once it gets going, for better or for worse.
Evangelicals tend to be highly distrustful of "tradition," as their origin stems from a reaction to the "religious maturation," aka liberal theology, of the late 1800s. In contrast, a group of scholars wrote a book called The Fundamentals, in which tradition was eschewed for a sola fide / sola scriptura basis. While fundamentalists sought to escape the culture, evangelicals intended to engage it- but in both cases, this reaction was grounded in an anti-establishment fervor. We basically threw the baby out with the bathwater. Anything "traditional," including liturgies, lectionaries, and church "holidays" were all suspect. Christmas and Easter survive, as much out of their popularity as anything else - but pretty much everything else was jettisoned.
(By the way, in my seminary, I remember a preaching class in which the professor had to remind us that Easter really was a special day and needed to be treated as such. That such an admonition needed to be said speaks volumes about what we think. )
As a result, Pentecost tends to be forgotten the "lower" church one goes. It's sad, because Pentecost is probably the most significant holiday after and Christmas, and it's 100% scriptural. (indeed, it even prefers Christmas and Easter in the Bible!) Still, there's just too much tradition against it now for things to change.