Because it's common enough that you can trade regular items for it and rare enough that it actually has some value.
If you wanted to trade in Tir it would be impossible to have enough to make a typical trade. If you wanted to trade only in Ber or whatever then it would be impossible to trade for many items as they are only worth fractional Ber.
It's not the only currency though. Less valuable items go for gems, more valuable for Ber or Jah, and even more valuable for small charms
In early years, due to the massive duping of highrunes, the Ist rune was the highest accepted trade rune in some legit communities (for example planetdiablo.de). It was possible to be obtained from The Countess or The Hellforge and it could be cubed with reasonable effort. The mf bonus was very important back then, that's another factor, rare uniques and rares had a lot of worth, so many people wanted Ist runes to actually socket them into stuff. Hard to believe.
For low items I also accept: Perfect Amys, Ral, Hel, Tokens, Keys. Please don't offer runes lower than Pul (with the named exceptions).
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Rhaegar86
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