Multichannel seller must have noticed what sells on eBay necessarily does not mean it will sell on Amazon and vice-versa. There could be various reasons.   

Familiarity:

Some are familiar with fixed price on Amazon and go for it as they normally do rather than waiting for the bargains on
eBay auctions.  The search results on Amazon are lesser than eBay since Amazon shows result by the product and then you can look to buy from different sellers whereas eBay shows result by all seller and you can decide on various criteria like prices, P  & P, Seller Seller Status.

Buy Box:

There clever “Buy Box” invention works very well on Amazon providing this facility to the good rated sellers with lower price of the item. Buyer buys without having to go through rest of the sellers. This speeds up the buying process. Absence of this facility on eBay may be a good reason when something sells better on Amazon.

Customer Type:

Delivery promise fulfilment is another factor that affects sales. Most of the eBay customers asks “Where is my item ?”
even if dispatch emails are sent and they are made aware on the listing the expected delivery time.  This normally does not happen on Amazon. Probably Amazon customers are more convinced on Amazon delivery time compared to that of
eBay. 

Experience:

Experiences buyers who had negative experience with eBay has good opportunity to switch to Amazon. Unlike eBay
Amazon has their own Warehouse for stock and dispatch orders themselves which is very important factor to build up trust among the buyers.

Product Reviews:

Amazon has user generated product reviews that speeds up the sales thought process resulting sales unlike that on eBay. 

These may not be good reasons for selling on Amazon but the fact is the Amazon is growing more that eBay and don’t forget more stores = more sales. If you can, always try to add sales channel. It does not hurt to try Play Trade, Asos Market Place or eBid.