Back when I bought my SE, it was cheaper than getting a 6, despite having better specs. I also check for deals online a lot, and the SE was almost constantly being discounted. I think it's not unreasonable, based on these, to assume that it never really sold particularly well.

Based on that, I'd say that they stopped making the SE simply because only a very small, specific slice of the market actually wants to purchase such a device, and it's not worth keeping a unique product in their portfolio if the sales numbers just aren't there.