You do not need a reason to include disabled characters in your story. Able bodied people are not the default. Saying you need a reason to make a character disabled is like saying you need a reason to make them a woman, or a POC, or mogai. Or, conversely, that you need a reason to make them able bodied, a man, white, or straight. In fact I’ve started trying to make all my characters disabled queer WOC by default and only making them able bodied etc when the plot requires it and I can see no way this is not just as valid a storytelling approach.
Now, able bodied people are the majority, but disabled people are still 10% of the population. So they should be at least 10% of your cast. If they are not without good reason you are perpetuating ableism. Making your cast 100% ablebodied is a choice, and an unrealistic one at that. (Making your cast 100% disabled is also kind of unrealistic, and is one reason I don’t do it. But I don’t see that it’s any worse) EDIT: This depends very much on the context of your story. If it's set in a nursing home the number of disabled characters should probably be higher than if it's set in a unit of Navy Seals.
And yes, when you write those disabled characters, you have to write them as disabled, you can’t just gloss over their impairments and write them as able bodied. And it will be a challenge. Suck it up. Use it as inspiration to write a more interesting story. Or, if your story really doesn’t work with that disability, give them a different one or make them able bodied. But don’t give up before you’ve even tried by default.
There is absolutely nothing wrong with starting out with the intention to write more disabled characters, and it is not pandering. Pandering to who, disabled people? OH NO.
You know what I find insulting? When a character in fiction shows up with a disability and I KNOW it will be plot relevant (and probably faked as part of them being the villain) because god forbid disabled characters just exist without it being a big deal. Like we do in real life.