First let's define fallibilism as the view that there's no belief that can't turn out to be false no matter how much credence we lend to it.
This implicitly entails that we take ourselves to not be omniscient. If we are not omniscient there always may be defeaters for our beliefs that we haven't yet considered or that we maybe can't even possibly consider because they are somehow inaccessible to us.
Now we have no clue what the nature of these defeaters is, if they exist, what the probability of them defeating any given belief is etc. So basically every judgement we make has a variable attached to it, that can basically change the result of the whole calculation. And that applies to all beliefs simultaneously.
If there aren't some beliefs we take to be infallible, wouldn't total skepticism (as in global suspension of judgement) be the necessary result of the fallibilist's commitments?
I'm therefore suspecting that describing fallible beliefs as constituting knowledge is a pragmatic decision more than anything else.