We live in a world that has been transformed by the digital era. Everything at our fingertips and for a fraction of the cost. So, why would I prefer an actor in a costume or even animatronics to CGI? Simple! The performance of the the actors. I feel that their performance to something that is physically in front of them is far more convincing than a performance against something that isn't there. With CGI, the actor must imagine what isn't there and then react to it. Pretending to be scared of a monster that isn't there is much more challenging than freaking out at the guy in a creepy costume. It is much easier to look at a suited actor than it is something that isn't there. Yes, markers are placed so the actors know where to look, but sometimes the final edits end up a bit off from what was predicted during the shoot or the actor misjudges the size of the imaginary creature and in the final film, they aren't really looking at it because it is placed differently or is bigger/smaller than anticipated.
I also feel that the time spent creating the costume/animatronic will carry into the production of the film. As a director, I feel you are much more likely to really judge the performance of your actors to ensure that you are getting their best performance. If you spent hours or even days creating your monster, are you really going to let a performance that isn't 100% slip by? Probably not. You just spent all of that time creating it so now you want to make sure that time was well spent. You will work with your actors to make sure that they give you their best performance.
Now, I'm not saying that CGI shouldn't be used in films. There are plenty of films where CGI makes the special effects easier or the entirety of the creature couldn't have been captured any other way. It's probably way easier to use CGI for a horde of spiders than trying to make a bunch of animatronic spiders. All I'm saying is, it doesn't need to be used for everything. If you can use a man in a suit or animatronics, then why not do so? You will have a much smoother performance from your actors and everybody will be looking where they should.