1. Theme song: Aside from being a truly hilarious show, another reason why Friends became so popular is that it had a really good theme song. Some people even claim that, at one point, the Friends theme performed by The Rembrandts was more popular than the show. Though Joey's theme song isn't bad at all, for me, it lacked the hooks that made "I'll Be There for You" such a catchy song.
2. Character build-up: One of the secrets of Friends's success lay in its pilot. The writers created brilliant expositions for the characters of Phoebe, Chandler, Monica, Ross, Rachel, and Joey. Their characters were built up and formed so well that by the first episode, audiences were already well-acquainted with each character. They already knew from the start each important facet of every character at varying but generally equal degrees. This is not case with Joey. Audiences know Joey better than Gina or Michael. Audiences, assuming that the majority of them are former Friends viewers, have known Joey for 10 years. This is will pose as a problem because Joey will now lack the ensemble cast magic that made Friends such a hit.
3. Bittersweet feeling: I don't know if this is a valid reason. I don't know if this is a universal sentiment that most Friends fans have while watching Joey, but I won't enjoy watching Joey as much as I used to enjoy Friends. I mean "used to" because I no longer enjoy Friends the same way as before. Let me explain. Joey, as a sitcom, is good, and this is but expected because the spin-off inherited Friends's brilliant writers. These writers have, by now, mastered the art of writing for a sitcom. These writers are so good that, with the help of LeBlanc, some scenes in Joey end up bringing back the atmosphere of Friends. Though it may seem as an accomplishment, it doesn't work well for me. When those Friends moments occur in the show, I jolt with a sudden feeling of happiness which quickly dies down into sadness. I start missing Friends. In the same way that I get sad watching Friends re-runs and in the same way I don't enjoy watching American Idol as much as before because I miss Season 3, I can't fully enjoy watching Joey because I am constantly reminded of what no longer is and no longer exists.
4. Reality TV shows- We were all sad (at least some fans were) to see Friends go, but we all knew it was for the best. We didn't want it to end, but we knew that the show already reached its peak, and it had nowhere else to go but down. We would rather see it go while it was still quite hot, than see it as a fallen TV hero, ending it's run when no one wants to watch it anymore. You see, we have entered the era of reality TV shows. I have nothing against them. In fact, I follow some of them, but I have to admit that some of these shows are getting out of hand. Anyway, the influx of these reality TV shows, which probably began with the first season of Survivor, has slowly pushed sitcoms down in the Nielsen ratings. If I'm not mistaken, Joey premiered as the highest-rated sitcom, but it was not the highest-rated show. Maybe Joey arrived at the wrong time. Joey arrived during sitcom's slow exodus. Sitcoms will never die, but as of now, I doubt that a sitcom will reign as the most-viewed TV show for some time.

The fact is that it's not really helping that Joey is a spin-off of an uber-popular sitcom; it's actually hurting it. Basically, Joey is a good show for someone who never followed Friends faithfully. If you have no idea what Central Perk is, then maybe you'll enjoy Joey more than me.