The simple answer is: it's what Amazon had in stock at the time I purchased. I have not heard that going thicker gauge (10 vs. 14) would present any problems as long as the cable purity and manufacture was good, which BJC's is. The issues I've heard is going the other direction, going with too thin a gauge that might not be able to handle the power being output by the amplifier.
Regardless I don't think there's a substantial savings in going with BJCs 12 or 14 gauge options (I haven't checked, so I could be wrong).