If you are musically inclined, you know why; if not, that's because C-major is the simplest key: The notes are "whole," i.e. without any sharps or flats necessary in the notes/chords.
Um. Anyway, so we had the song in C, which is easy to play. Thing was, it was a vocal stretch for most everybody in that key. I could sing bass at the bottom of my range, but the high end was way up there.
So, the solution was to rearrange the piece into another key, and the pianist suggested E-major, since that would be down (or up) enough steps for us to hang out comfortably.
Easy for a pianist to say. If you are playing a piano, you can go off into to sharp- or flat-land, just use the black keys, but if you are a guitarist, or strumming a ukulele, the farther away you get from C, the more likely it is you will have to use a finger-busting chord.
No sharps nor flats in C, but four-sharps in E.
If you are good guitarist and have the barre chords all the way up the neck, you can adjust keys fairly easily–move everything up or down, stay relative, no problem; however, if you are still stuck in what are call the "cowboy chords," up in first position (that's near the little tuning pegs and gears and all), then shifting from a C#minor chord to an F# to an E can get to be tricky in a hurry. Most of the guitarists and uke players where I jam still use the cowboy chords most of the time.
You can use a capo, which allows you to play different keys using simpler fingering. Play it in C, capo up four, presto! E! but the uke players don't have 'em.
So I compromised and redid it in G. I think most of us can get it there, and there only one sharp in that key.
We'll see how it goes ...