Because they literally didn't even look.
From the accident report:
"It would have been technically feasible to include almost the entire inertial reference system in the overall system simulations which were performed. For a number of reasons, it was decided to use the simulated output of the inertial reference system, not the system itself or its detailed simulation. Had the system been included, the [design error] could have been detected."
Mainly it looks like the decision not to even TRY to test with Ariane 5 data was a cost decision... A "just use the system from Ariane 4 and it'll be fine..." type of decision. No rational thought behind it.
The Ariane 501 mistakes were massive...
- Code/system reuse without ANY simulation whatsoever with expected flight data numbers that were FULLY expected on the brand new platform.
- Designed to fail even prior to that mistake... both IRUs could shut down leaving the platform literally without the data they provide.
- Running code in flight that was no longer necessary AT ALL during that portion of the flight. Code that was left running on Ariane 4 for CONVENIENCE of not needing 45 minutes to re-align the platform after an on-pad abort, but completely unnecessary aboard Ariane 5. The unnecessary code was what crashed the IRUs. NONE of the data they were outputting was being used after liftoff.
- Code that outputs TEST data when restarted, running on a PRODUCTION/FLIGHT platform, and nobody writing into the RECEIVER to IGNORE that data in-flight.
(And more. So many errors in that accident, it's a horrible example of "complex" systems... it's a great example of "ASS-U-ME-ing" old stuff will work on a new platform.)
You also get THIS kind of crap in aerospace, especially space... and various folks have pointed out that accident reports often gloss over or don't contain ANY real data about what "tests" were actually done... and VERY few companies will allow third parties to look over their ASS-U-ME-tions unless the project management DEMANDS it. Not open cultures. Secretive. Because... rockets!
"The MCO report contains little information about the software engineering practices but hints at specification deficiencies in statements about “JPL’s process of cowboy programming” and “the use of 20-year-old trajectory code that can neither be run, seen, or verified by anyone or anything external to JPL.”"
A pretty decent analysis of a large number of space losses incurred, caused by software, from an MIT prof who specializes in such stuff... if you're bored. It's actually a pretty fun read if you like stories of how software engineers and software engineering really works in the real world... and remember these are hundred-million-dollar plus projects... it's not a resource issue, most of the time. Smaller companies stand NO chance of ever getting software right, if these folks can't.
http://sunnyday.mit.edu/papers/jsr.pdf
Conversely, the Shuttle had a big "win" in software in this regard... one of the most professional groups of software writers yet documented on the planet... not cheap, not sexy, not flashy, but damned good code. And they lowered their error rate by 90% after they started, an unheard of number in software development.
https://www.fastcompany.com/28121/they-write-right-stuff
"That’s the culture: the on-board shuttle group produces grown-up software, and the way they do it is by being grown-ups. It may not be sexy, it may not be a coding ego-trip — but it is the future of software. When you’re ready to take the next step — when you have to write perfect software instead of software that’s just good enough — then it’s time to grow up."