Either the Indian investigators did not like what they found (it implicated the crew, ground personnel, airline, etc rather than the plane maker) or they were unable to understand the data and were not willing to admit this and hand the boxes over to the experts.
Why would I presume this?
Simple. They've announced the recovery of both boxes, so we know they have all the flight data and voice recordings.
Unlike you, the various government agencies that investigate crashes have a responsibility to make sure they have everything correct. Analyzing the data is not instant. People seem to think investigations happen like in CSI. Determining the root cause could take months. Part of the investigation is determining if it could have been prevented, what steps can be done to avoid the cause in the future, etc. The crash happened on June 12, 2025. It has not been a month since it happened, yet you expect findings to be released immediately?
If there were data recorded to indicate a problem with the design or construction of the plane, there would have been immediate notifications to the American and European air safety agencies and the plane manufacturer, which would have been followed by airworthiness notifications and possibly groundings.
Your assumption again is that somehow investigator instantly know the root cause and are ready to release their findings. When the investigation determines that it was a design or construction problem, the appropriate agencies are notified.
When there's a known safety issue with the design of an airliner, the various safety agencies are not just gonna sit there on their collective hands.
Part of the investigation has to take into account how it has to be done. Suppose the root cause is a faulty design of a part. It will need to be replaced; however, the replacement part has to be redesigned first. Then it will take time to replace the part on all affected aircraft even if all the parts existed. Things do not happen magically.