Most likely since it is a custom build, it would not have the same amount of testing (a lot of bend-tests are done to wings and I believe whole aircraft on commercial airliners) and probably a lot less simulation and engineering.
Since it was designed as a shuttle transporter, they probably designed for 2000 landings and never cared about really defining safety margins beyond that. After all, 2K seems plenty for even an ambitious shuttle program.
Probably in reality it would be fit for 50.000 flight hours, but no one knows how much, so they implemented huge safety margins - since the USSR was not a market economy, no one really cared about price in such a high-profile project.
And: the Antonow has a max take-off weight of 640 tonnes, the 747-400F of 400t, that would put a lot more strain on materials.
Since it was designed as a shuttle transporter, they probably designed for 2000 landings and never cared about really defining safety margins beyond that. After all, 2K seems plenty for even an ambitious shuttle program.
Probably in reality it would be fit for 50.000 flight hours, but no one knows how much, so they implemented huge safety margins - since the USSR was not a market economy, no one really cared about price in such a high-profile project.
And: the Antonow has a max take-off weight of 640 tonnes, the 747-400F of 400t, that would put a lot more strain on materials.