When is a lower gain transistor preferable? - Letter suffix on bipolar transistors
The reason would usually be price.  The manufacturing process for transistors results in considerable variation in gain.  The manufacturers test for this and "bin" the parts according to gain and possibly other parameters.  They can sell the better parts for more money.  So you chose the C grade when you need high gain and the A grade when the lower gain is good enough and you want to save a little.  In high volume applications the little savings can add up.