Excellent question
The two viruses are very similar but MERS has a larger genome than SARS. In addition, all viruses have to bind to a particular protein on a host cell; SARS binds to a protein called ACE-2 and MERS to a very different protein called DPP4. Both of these proteins are found in the respiratory tract. SARS-CoV-2 also binds to ACE-2.