Why does Lowes sell L grade copper pipe cheaper than M grade?

Why sell M at all? L-pipe is nearly 50% thicker wall in 1/2" pipe, it's tougher and easier to live with.

If L is cheaper, leave the M exactly where you found it.

At least on their website that's not necessarily the case.  Make sure you are comparing the same diameter pipe.  Otherwise my guess would be they don't sell much M type pipe since it's not supposed to be used for plumbing applications so it ends up being more of a low volume specialty item.