Why Do Online Brokers Only Offer Small, Random Amounts Of Shares To Short?

I have attempted numerous times to short with Scottrade, and they often say “only 23 or 25 or 37 shares available to short.” These were for heavily traded company shares (in the 10s of millions).
How is it possible that these firms only have such small, random amounts available to short at any one time? [...]

Can Anyone Advice Me On The Best Online Broker To Choose? I Have A Short Term Trading Strategy.?

and some possible day trades. I need one with trailing stop/loss capabilities. Also, I’m trying to figure out if I need stock trading software? I have the specific parameters (Tech A.) mostly, so what is a good software? does any of the online brokers offer a good “real-time” software as well [...]