The Iron Yuppie

Anyone ever wonder what the etymology of the contraction "'Tis" is? I mean it's the exact same letters and characters as "it's" and, if I understand it correctly, it's the exact same abbreviation. The odd thing is that I assume "'tis" was before "it's" so why did people switch over when the time came? People still say Halloween (all hallow's eve) or other archaic abbreviations. So curious...