Because blocksize limit was lowered from 32 MB to 1 MB by Satoshi in early days of Bitcoin. He figured out that without such a limit the blockchain would be bloated and Bitcoin would be abandoned. Also, if u read the forum u would see a lot of threads discussing that without the limit Bitcoin will die. It's because transactions must compete each with other (by paying larger fees), otherwise mining will become unprofitable. There are some other "why"s, but I can't write in a few words so many threads, use [Search] button, plz.