They were used a lot because every
Jewish man had to pay the half
shekel Temple tax every year, and
had to pay it in that one currency. That adds up to a lot of shekels.
Judea was relatively
poor, with no internal sources of silver, so most people probably didn't use silver coins very much apart from that. Remember the stories about the widow with nothing but two small copper coins, and the woman who searched the whole house for a lost silver coin,and threw a party when she found it? It's not
good enough to say 'It was a common coin therefore they paid Judas with it'. You have to look at what it was and what it was used for.
The only mention of the payment is in Matthew, who says '
oi de esthsan auvtw triakonta avrguria', 'They paid him thirty silver'. The only other place in the New Testament where '
arguria' is used is in Matthew (he seems to like the word) 25:27, where it clearly means 'money'. The word is used four times in the Septuagint, and only one of those has any possible connection with shekels. Zechariah 11:12 is often translated as referring to 'Thirty shekels of silver'. It may well be where Matthew got
his inspiration from, since he's virtually quoting it, but unfortunately shekels are not mentioned in either the Hebrew or the Septuagint, so they're something the translator has added. It's not in the old translations as far as I can see, and makes its appearance in Victorian times. I'm normally not in favour of using the
King James version, but it does sometimes get it right!
It's commonly
asserted that the coins were shekels, but nobody that I can discover has come up with anything to back it up at all.