Long story short: changing a bit from 1 to 0 is really easy, but changing it from 0 to 1 is quite difficult and requires an expensive erase operation. The erase works on a full 4k block, so writing a random byte means reading 4k to a buffer, changing one byte, erasing the page, and writing back 4k. Sequential writing means erasing the page once, and writing the bytes in known-pre-erased sections.

Any modern (last few decades) SSD has a lot of sauce on top of it to reduce this penalty, but random writes are still a fair bit slower - especially once the buffer of pre-prepared replacement pages runs out. Sequential access is also just a lot easier to predict, so the SSD can do a lot of speculative work to speed it up even more.