They rolled the dice that day -- 1 in 4 chance that a leak would burn through a strut -- and lost.
But if the leak had been on the outside of the booster, would they have just continued to fly until some future mission failed in the same way Challenger did?
They had had burn-thrus before, but the gasket melted and flowed into the gap, sealing the leak, as it had previously, and as it was designed to do.
Here is the sequence:
1. It was an extremely cold, wet winter.
2. Technical problems kept Shuttle on pad longer than usual.
3. Freezing weather at launch time.
4. There was an SRB burn-thru on Ignition as had happened before
5. After a second, the gasket flowed and resealed as it had numerous times before.
6. No more problems until throttle-up after Max-Q.
7. Then just as they throttled back up to 104% power, the highest upper air wind shear ever recorded buffeted Shuttle and rocked the entire stack.
8. SSME gimbaled over to correct
9. But rocking the stack reopened the previous burn-thru.
First Bad Luck:
Extreme High Winds at Throttle Up
Second Bad Luck and Primary Cause:
Burn-thru was located on the minority portion of the arc opposite the ET (External Tank). If the burn-thru had occurred on the ~ 300 degree arc not opposite the ET, the shuttle would have achieved orbit with no problem. The slight loss of thrust from the SRB burn-thru would not have been a problem.
It was only sheer bad luck that the burn-thru occurred where it did, no other reason.
And don’t even start about how the SRB’s were segmented to start with. NASA wanted to build the SRB at the Cape, but politics dictated that the Shuttle project largess be spread around to other states, including Utah where the SRB’s were built.
Unfortunately, due to a non-segmented SRB being too large to be transported, due to tunnels and overpasses, they were segmented.
Otherwise, no segments, no burn-thru, no explosion.