1. Question: Why can’t we say a shehecheyanu on making matzah but we could say a shehecheyanu on making the Sukkah?
A. The Magen Avraham (641:1) says we don’t make a shehecheyanu on making a shofar and megilla because these are mitzvos that could be used for many years.
B. Tosfos in Kesubos 72a “visafra lah” says that a woman who has a flow (zavah) does not make a blessing on her counting of 7 days though it’s a mitzvah to count like the omer because maybe her count will get messed up with seeing blood again within 7 days.
Answer: We don’t says a blessing or shehecheyanu on making matzos because maybe they will become chometz in the process and won’t be able to be used on Pesach just like the zavah whose count of 7 days might get ruined. (See Dirshu Mishna Berura 453 footnote 26.)