Has not sold suggests that product #55 is still available for sale.
Here's a typical example:
My house has been on the market for two months. Why hasn't it sold?
This implies that the house is still on the market. The speaker made the house available for sale two months ago, and no one has bought it yet. The speaker is still trying to sell the house.
Didn't sell suggests that the period when product #55 could have been sold is now over.
Your house was foreclosed but didn't sell at auction.
This means that the house was placed for sale at an auction, no one bought the house, and the auction is now over.
Another situation where you might say that product #55 didn't sell is if it's a seasonal item and the season is over. For example, if product #55 is a winter coat, and it's now summer, you might say that product #55 "didn't sell": it had its chance in the marketplace, and it failed.
In this context, didn't sell can mean "didn't sell enough to be considered successful". The simple past suggests that we have enough evidence to conclude that product #55 was unsuccessful. We know. The present perfect has not sold can suggest that there may still be some way to make product #55 successful or perhaps you need to allow more time for sales to pick up. However, if product #55 has been on the market for a long time, has not sold can mean that you are suggesting that now it is time to give up.
The difference in meaning comes from the fact that the present perfect tense locates the action in a time interval that starts in the past and reaches at least to the present, while the simple past tense locates the action at a certain time in the past. When no time is specified with the simple past tense, there is usually (not always) an implication that the event finished in the past and is not continuing in the present.
For more about this, see my answer to this question. (Note that not everyone agrees with the time-interval theory.)