Theories on why Piccolo stayed on Earth:

*While waiting for the Earthlings to wish the dead people (Tenshinhan, Chaotzu, etc) back to life.

Dende: Hey Nail!

Piccolo: I'm Piccolo.

Dende: You'll be coming back home with us right?

Piccolo: ... Just out of curiosity, what are you guys gonna do there?

Dende: Why, Nail-

Piccolo: I'm Piccolo

Dende: We'll be rebuilding our new planet, of course. Planting those trees to make our planet as beautiful as it once was.. Living happily and peacefully together.. Then

there is the matter of redistributing the dragon balls again.. And you can resume your old job again, Nail!

Piccolo: For the last time, my name is PICCOLO!

Dende: Sorry, it's just that you look so much like Nail..

Piccolo: Yea, well so do the other Namekians.. Let me weigh my options first.

Piccolo starts writing a list of pros and cons:

Stay on Earth:

1. There are many other people stronger than you who can beat you to a pulp.

2. You can never walk freely in public without anyone gawking at the sight of you.

3. You will be the only Namekian there on Earth.

4. Earth seems to be a dangerous place. You can expect new, evil, stronger opponents in the future.

5. You will not have a job on Earth.

Go to new planet with Namekians:

1. You will be the strongest and can beat everyone else to a pulp.

2. You can walk freely in public and no one will gawk at you.

3. You will be amongst your fellow Namekians

4. The new planet seems to be a peaceful place. You can expect a safe environment with peace and happiness.

5. You will be entrusted with the important and honourable duty of protecting the new Elder, the dragon balls, and your new home.

Piccolo sinks deep in thought. Dende then looks over at Piccolo's list of pros and cons.

Dende: Wow, looks like you'll be going to the new planet Namek with us huh, Nail?

Piccolo stares at Dende in silence.

Piccolo: Well, I've made my mind. Definitely Earth.

Dende: Okay then. I'd be happy to meet you again should fate allows it. Bye, Piccolo!

Piccolo: It's NAIL! I mean, PICCOLO! I mean-Grrraaghhh

Dende then wishes all of the Namekians except Piccolo to the new planet.

After the Namekians are gone, Piccolo reviews his list again. Glancing around suspiciously, he then scribbles an additional item to his list and grins.

Stay on Earth:
6. Gohan is here.

Go to new planet with Namekians:
6. Gohan is not there.

*Epilogue:
In Piccolo's mind:

Kami: "Good choice in staying, Piccolo!"

Piccolo: Huh, oh. It's you.

Kami: "I sense the evil fading away from you. I am glad you chose to remain here. The Earth needs a strong protector such as you. And honestly, I'd feel safer being able to know your whereabouts. Why, I might even feel lonely here-"

Piccolo: GET OUT OF MY HEAD, KAMI!