The choice of a uint32_t for time saves 4 bytes per transaction. That doesn't sound like much, but with 1.2 billion transactions recorded, it adds up to almost 10 GB of space saved.
They could, ultimately, just replace it with a uint64_t some time in the 22nd century without much fuss. In the late 2000s when Bitcoin was created, storage space was at a significant cost, but now it is quite cheap and in the 2100s it will undoubtedly be even cheaper.