One was released last September and the other was released a few months later as a standalone rather than an expansion. Hexcells Plus actually has a bit more content, but the puzzles are entirely different. I'm not sure why it was done this way. I suspect that the first was just a bit of an experiment by the developer and it garnered enough sales/interest that he wanted to expand on it a bit, but he had not built any way to add content into the original.

Why not buy the pack?

Both games are very good. You should go for twopack of Hexcells. More fun, more puzzle.