A future mathematical weakness in RSA/DSA could have been a concern, but 2 larger reasons to use EC in BTC are that a) signature computation is much faster, b)

A future mathematical weakness in RSA/DSA could have been a concern, but 2 larger reasons to use EC in BTC are that a) signature computation is much faster, b) almost every 256-bit value is a valid key, allowing fast key generation that supports the (often ignored) recommendation to use a new key for each transaction.