Data loss happens in two ways. Either the files are erased or corrupted (say by a worm sweeping through and munching on things), or by the physical media on which the files were stored being damaged. The worm didn't erase or corrupt everything in its path, but it did enough damage that what remained in affected systems was of questionable use. And facilities that held the data then got hit with the fallout from where the worm did take out critical safeguards. Power systems malfunctioned all over the planet, resulting in power surges and blackouts, fire suppression systems going off, water mains bursting, and all other kinds of chaos. Then you have the metahuman element, where all it takes is one person breaking a window to steal a new trid for their apartment, and suddenly you have a citywide riot. Hell, even today there is always looting when you have blackouts and communication outages.
As for how Renraku's data stores were spared in Asia, I'd assume that in addition to extensive firewalls and security deckers which would have slowed the worm down, they'd have some way to cut the hardline to the servers, or even cutting power to the entire facility (which would cut the servers off from the Matrix, naturally). This is not as simple as it sounds, but it could be done fairly quickly, though getting systems back online afterward would take time. But the data would be safe, as long as you had enough warning to isolate the host and cut it off. The Worm didn't hit everywhere all at once, afterall. There were eggs spread around the world, but even so, there's a limit to how fast the worm could move through systems and replicate.