I'm sorry I don't have a link for this, since it's been a long time since I read it (and may have read it off the internet). Ron Moore, one of the writers of that episode (and, at that time, one of the co-producers) has said that they did have an explanation for this written in the script. Originally there was something about "surface tension" and how they didn't go through walls unless they pushed, so as long as they didn't push too hard on the floor, they wouldn't go through. And notice, due to gravity, that it would be easy to push against a wall, but not against the floor (without jumping on it).
The problem was production realities. In short, it was in the script, but didn't make it to the final print of the episode.
This also brings up some interesting points and issues regarding television and film production. On this site most people that ask questions about what is going on inside a science fiction or fantasy story want an in-universe explanation and sometimes people give answers like, "Because the writers are stupid," or, "The writer wanted it that way." I can understand the 2nd, but the 1st is not only a poor answer, but is invalid. Especially when it comes to TV and film.
Few know just what it takes to create even the 1st draft of a screenplay and even fewer know what it's like getting that screenplay into something that can be produced and produced on a budget. This can be a factor in film, but it is always a major factor in television. You need a new script every single week. It has to be a compelling story and you have to keep people interested for 60 minutes and through 4-5 commercial breaks that can last over 4 minutes per brake. The story has to be interesting enough that people are willing to waste over 25% of the time they're watching it on being assailed by commercials.
A TV or film script goes by at about a minute per page. So a script for an hour show is 55-60 pages (or a little over that). And it has to be divided into acts, sometimes 5 (like in ST:TNG) but usually in 4 acts. You have to have a cliffhanger at the end of each act and have to watch for pacing, so you don't drag, or have slow scene right after a fast paced on. You need to be sure there is enough time in the script to allow characters to interact and if you're dealing with long story arcs, you have to make sure you include scenes to the long arcs.
An idiot cannot write a TV or film script that will be produced. It's not an easy thing to do.
But, beyond that, you have re-writes. A freelancer may turn in a script, then the story editor, the producer, and even executive producer will make changes (not "may" or "might" -- they WILL make changes). And then there's the read throughs, where it's possible they find scenes or lines that don't work. And even when the script gets in production, changes are still being made for many reasons. (Maybe they don't have time to do an extra setup in the conference room, so they have to change a scene to the ready room, and that means other changes -- or if they don't get a scene shot today, they may not have the actor tomorrow and need to write them out of a scene.)
And that carries us only until the filming is complete. FX can create issue and then, once all the footage is complete, it has to be edited. And a scene that timed out in a reading as 1:30 might run 2:05, with no way to cut it. That means 35 seconds has to be cut from somewhere else in the script and someone (rarely the writer) gets to pick what will be removed to make it work.
Throughout each step, the writer, producer, editor, director, and even other people, have to take a lot of factors into consideration. Is the pacing working? Is that scene believable? Did that line that looked good just plain suck when the actor read it aloud? Is the script running too long or too short?
If it's long, what do you cut? Do you cut a beat in the plot point so you have time to explain why Geordi doesn't fall through the floor, just so about 1% of the viewers will get that point? Or do you cut out the explanation that only 1% will care about in the first place?
Making movies and TV shows is complex. It is extremely rare that a script comes out shorter than the allotted time for the episode. (One of these very rare cases is the famous and excellent episode of The Mary Tyler Moore show, Chuckles the Clown Bites the Dust.) So things have to be cut. Some by the writer, but most by other people along the way. Each person editing the story will have a different eye and different concerns, so no one person could be blamed for an omission or error in a script. This question brings up just one example, but I'm sure, with a little thought, we can all find many others.