Simple answer.

The product was very unsuccessful and ran a limited commercial showing during debut sales.
The commercial ran in cheap ad spots at odd times because the startup lacked investors.
After only a few showings and very few orders, the company could no longer finance televised commercials.
Due to the fact the product was a risk for sales, no store ever started carrying it, and it was available by phone orders only.
By the time spongebob recieved his in te mail, the company had already folded.

Poor business practices and underfunded marketing were the reasons nobody knew.

Meanile, Kelpo is very successful and can afford to run daytime and primetime ads on national circuits.