Acting as a short-term store of value is very important for a currency, but it's bad for investment if it's good as a long-term store of value. Money at its core is not a store of value; that's one of the requirements, but at its core it is a common medium of exchange. It is the very indirection that makes markets using money more efficient than ones based on barter; having a single common medium of exchange means you only need to solve the price discovery problem for n products rather than n^2.

Deflation is bad because it turns money into an appreciating investment asset; this means it reduces productive investments (i.e. those that actually incentivize people to create wealth). A (quasi-)fixed base of money as a root cause of deflation lets people with a lot of money to begin with siphon off wealth from people doing productive work that increases the size of the economy and the velocity of money.

TL/DR: deflation means theft of wealth by people owning currency at the cost of the economy at large that uses the same currency.