Maxim were the first to sell the MAX232 so they are best known. As a result, they can charge more and still sell enough to make a good profit.
There are lots of different types of MAX232 clones. There are significant differences. It can be worth paying more for ones that work at lower voltages (down to 3V as opposed to 5V +/-10%) or ones that work with smaller value capacitors, which are smaller and cheaper.
There is also the skew rate. If you have a lower skew rate, there will be less electrical noise, but lower maximum baud rate.
Some MAX232 clones regulate their output voltage, which can result in much lower power consumption.