Why Matsya and Kurma avatars are not worshiped like other avatars? Vamana as trivikrama, varaha has temples in south. Rama, Krishna, Balarama and Narasimha avatars are worshiped and many temples are there across India. A separate question was asked about Balarama avatar. Kalki is yet to come.
The Matsya avatar incarnation was for fetching the stolen Vedas basically and to patent-save the prototype of Srushti-materials by that period Manu, as the Kala (time) was nearing Pralaya by deluge. The Koorma avatar was for providing the Devas their lost glory. As is evident from these fundamental causes, both avatars were not for humans but for celestials ! Hence why should humans worship those forms? Also Varaha avatar is concerned directly with Earth, our dwelling place and evolutionally fits in chronologically too as a starting point until today..! The Lord Varaha has also given instructions through his words called 'Charamasloka' as to how a man can stake his claim for attaining Salvation or Moksha..!