Why are most RGB LED strips common anode instead of common cathode?
The reason common anode is more common is because its easier to sink current than to source it. With either common anode or common cathode you'll have one terminal connected directly to a supply for all LEDs and the other side having the dropper resistor and a control transistor per pin (or IC outputs that are transistors on the inside) either sinking or sourcing a current.

NMOS / NPN transistors are stronger in general, more common as discrete and  are better at sinking current than sourcing. You need PMOS/PNP transistors to source current (pull up) effectively, but they'll still be weaker at sourcing than an equivalent N-transistor would be at sinking. Thus the best solution is to connect a common anode to the positive supply and sink current from each LED using NMOS transistors. 

Older ICs used to be designed exclusively using N transistors for speed reasons, and so were much better at sourcing current than sinking it. This was particularly true of the TTL logic used in the 74LS series chips (still widely used as interface chips). A 74LS00 is specced to sink 4-8mA, but source only 0.4mA.

Modern CMOS ICs are much more symmetrical (an ATMEGA328 in an Arduino can source or sink 20mA) since they use bigger PMOS than NMOS to balance the fundamental differences, but the convention of common anode is well established.

EDIT (More info): If on the other hand you're building a matrix, you'll have to have both current source and sink transistors. In this case it can be best to have more devices in common cathode and fewer on a common anode. The idea here is to have a few fat NMOS devices sinking many LEDs currents and many weak sources (I/O pins) driving a few LEDs each. Of course with common anode strips you could use fat PMOS devices too.