As you work at SGA, you accrue work credits. Those credits 'expire'.
Some of the more knowledgable posters can probably supply the details (like how long a credit is good for), but as an example.....
One could start working at age 16.....and work full time for 15 years. If they became disabled then, their SSDI would be based on SSA's formula for the years they worked (how many quarters/credits) and their salary.
If that person got married at 31, started having babies, and became at stay at home parent, then became disabled at age 51....they'd be up a creek with SSDI because they hadn't worked for 20 years, so their credits would have expired.
It sounds like your case may have gotten close to the deadline for your credits expiring.