Why do some RGB LEDs have six legs?

According to the datasheet, it has six legs because it contains 3 LEDs, with nothing common:

Many RGB LEDs have a common cathode or common anode, so need only 4 leads. For some applications that may not be acceptable. For example, it wouldn't be possible to control two RGB LEDs with common cathodes or anodes in series, or to arrange them for charlieplexing.

Further, a part like this is at least more flexible. It can be made common anode, or common cathode. Or, a pair of the LEDs may be arranged in anti-parallel. It may make more sense in some situations to stock just one part than to stock several different parts to cover different cases.

These RGB LEDs have six pins because all the connections are brought out individually.

You didn't ask, but others may wonder why manufacturers do this. The advantages of bringing out all connections individually over tying some together in the package include:

• One part works in both common anode and common cathode configurations. If the package needs to be big enough for thermal or other reasons anway, then there is little advantage in economizing on pins. There can be bigger savings in only having to produce a single part for different applications than in reducing pin count.

• There are other topologies beyond common cathode or common anode that such 4-pin packages don't suite at all. For example, it might be useful to drive a string of each color to get higher voltage, which may allow for more efficiency in the power supply. Or, you might want a different power supply voltage per color or at least one for blue and another for red and green. That can be done with common anode or cathode, but is simpler with individual connections. The fixed power voltage could go on the anode, then a low side switch on the cathode.

Before you linked the datasheet and explained that it wasn't common anode, I commented that it might be mechanical or thermal relief.

For both of those purposes, you want larger pins or more pins. Large pins take longer to solder, so if more pins fit easily within a rectangle surrounding the device you might as well choose more pins.

This device claims a total power of 350mW, which is starting to get hot to the touch, so thermal design is definitely a consideration.

Single-colour LEDs often have a "pad" underneath, but in this case with three disjoint LEDs there would be the question of which signal to connect it to.

An actual reason for the non-common anode, according to the datasheet's "Description": the three sub-LEDs are manufactured on different substrates with different processes (may not be true for all RGB LEDs). Therefore there are three small dies in the package each of which has two bond wires. It's probably easier to bond them out to different pins.

Final note: the orange LED is much brighter than the other two in this package, for the same current.