The reason that this is a syntax error is that [ isn't part of the shell syntax; it's actually a command. Originally it was just a symlink to the test command. It still is, but it's also a built-in command in bash and other Bourne-derived shells.
if is a shell keyword, but the shell sees if[, not if. Because it didn't see an if, it doesn't know what to do when it sees then. (Actually, it knows exactly what to do: print a syntax error message.)
...
A bit of experimentation shows that it's not quite as simple as I thought it was. I tried creating a command called if[ and putting it in a directory in my $PATH. When I type just if[ at the prompt, the shell asks for more input. I actually don't know what it's looking for, but apparently the [ character is specially treated by the shell. The shell just doesn't split if[ into the if keyword and the [ command (as you might reasonably expect based on how other languages work). (If I really wanted to execute that command, I could type \if[ or "if[" -- or give it a sane name in the first place.
In any case, that last part probably doesn't matter; adding a space character will fix the problem.
