If I were starting over I might get a job that pays me to think of things like this. But then I would have had to study biology. Anyway ...
We note those creapy-crawly things come with any number of legs. The centipedes and millipedes as one group, spiders with 8, beetle type with 6 and segmented body type with 6. As is likely clear I studied neither biology nor taxonomy. Lots of different exoskeleton types came onto land. When it comes to chordates there was only one and it had only four limbs. This is nothing suggesting this would be a universal rule. Nor did Wells consider this in War of the Worlds although nearly everyone else expects bipedal tetrapods.
It is not clear how to eliminate any form or configuration of life from eventually developing a technological civilization and we crunch numbers looking for their sitcoms. I would like to address eventually.
Certain things make it easier to develop a technological society. Easier should take fewer millions of years. Living on land is obvious for the use of fire and not having salt water short out the experiments that would discover electricity which is essential to the RF we are listening for.
There is a precursor to developing any technology. That is limbs to manipulate the environments. We quaintly call ours hands but lots of things will do the job. Whatever does the job they have to be free to do the job. They have to evolve in a way they are free to do the job.
Earth has had many species which have gone bipedal. There were entire classes of dinosaurs down to our birds. Reptile birds and bats reasonably qualify as bipedal. In these cases the other pair did not become manipulators of the environment.
Then there was the entire class of primates with varying degrees of dexterity from next to none to us. Of our fellow apes there have been many more than survive today. Unless they used tools we can't say much about their dexterity other than they didn't use it for tools which is another of those criteria for a technological society.
Lots of changes have to evolve just to get from four to two legs. Walking upright requires even more such as a sense of balance and a skull at right angles to the normal orientation so we don't look only at the sky and drown when it rains. Given all the changes required before evolving intelligence can add to survival the path to technology for four legged chordates has a large number of contingent steps.
Evolution should be faster if manipulative limbs take fewer steps. The changes that involve being able to walk and still have two free "hands" should be many fewer if the land animals start with more than four legs. By definition every species has a use for a way to get food that is higher off the ground. Using two legs to reach for it while standing on four or more is an obvious evolutionary alternative to growing larger, getting a long neck or growing a trunk or climbing or learning to balance on two legs. The one strategy for getting food high off the ground is reaching for it. Even we started climbing into trees to get the food.
Once some sort of "arms" become common there are more species which can chance down the rest of the path towards intelligence and technology. Therefore it should happen more quickly in an evolutionary sense and this should be the most common form for intelligent life in the universe.
Which begs the question, is there anything special about fish developing four swim fins? A different question is what prevents exoskeleton forms from developing lungs? Noting of course our reasonably clear distinction (what are snails?) between a hard shell and an internal skeleton on land is also chance.
I see nothing that might give an advantage to fish with only four fins over any other number. But unlike our creepy-crawly friends we all appear to descend from a single species coming ashore.
Anyway, consider such an alien species with maybe six legs from the sides of a somewhat squat body with say the second pair of original legs as arms rising above the body and a head protruding upwards from the body between the arms.
Now we know why the Krell doorways are shaped that way.