Several reasons, actually.
1) It makes the job of the opposing defense too easy. Even teams who don't do much pre-game prep (which I suppose can happen in this nutty season) will figure out by the 4th quarter where the ball is going when their opponent needs to score. Couple this with #2, and you have what happened in the Pacers game: the scoring stops just when you need it most.
2) Your stud gets tired late in the game, especially if he's also working hard on the defensive end. This too happened in the Pacers game (on both ends of the court). If/when Amare comes back (healthy we hope) maybe things will get easier for Melo, but the same effect can happen.
3) Melo lately has been very good about passing the ball around and not taking EVERY ISO play he can. But as in the Pacers game, and apparently whenever the pressure is on, he completely STOPS passing and takes every shot himself once he gets the ball in crunch time. This is natural, but very bad for the team of course, due in large part to reasons 1 and 2 (especially #1).
4) Yeah we have a scoring stud (2 with Amare), and yeah scorers score, but to beat good teams you need to get a certain minimum number of easy baskets. And there are just not enough of them if every play is an ISO. Put another way, the shots in an ISO-heavy offense tend to be, on average, lower percentage ones.
Btw... let's define what "ISO" is. I mean.... in the end, every play could be called an "iso" because every shot is taken by one guy. An ISO happens when one guy gets the ball in no particularly open position, everybody else is uninvolved, and that one guy must make his own shot happen, all by his lonesome.
Even with a scoring stud, this is not good basketball. If the stud is hot that night, sure you can still win games. But good defenses -- and even mediocre ones in crunch time -- will shut this down pretty easily. Especially when, as happened in the Pacers game, they know exactly what's coming on just about every play in the 4th quarter.
5) I can't be sure what's more important than anything else, but this one feels pretty important. When a team devolves into one ISO after another on offense, 4 out of the 5 guys on the floor are taken out of the game half the time. Tyson keeps himself busy (because he's an experienced and great player), but most of the other guys can be caught standing around, watching and hoping.
As we know, basketball, like most sports, is a heavily psychological game. If you're confident, you tend to do well. If your mind isn't focused on the game, you're gonna suck. When a game plan involves taking most the players out of the game half the time they're on the court, eventually (at least some of the) guys are going to lose focus. And interest. And involvement. Of course, this can and often is overcome -- but it's a natural tendency that comes with the ISO-heavy offense that can only hurt a team.
6) This is a team sport. Five guys working to achieve one goal are going to succeed more often than one guy. A well-tuned offense is a thing of beauty. If you slow it down, you can see EVERY PLAYER involved in the offense.... whether it's setting multiple picks, rolling or popping, providing a 2nd or 3rd or even 4th option in a set, passing the ball, or taking the shot -- everybody's involved. It's a lot harder to defend against that kind of offense than it is to stop one guy from scoring one-on-one, over and over and over again.
There's also the effect on/of the refs, etc...... but enuf's enuf.
Not having a real point guard is this team's biggest problem now. (BD does not count.) I just can't understand how, as Woodson was quoted saying, they could believe that this team is OK as is. I wish they'd have at least TRIED to get somebody else in to be an actual PG on this team. There are still some available free agents out there, and some guys who just finished in China. To me, not trying is equivalent to rolling over and waitin' around to die.