Apparently, it's due to a misinterpretation of the m'sora. By the rules of grammar, both should be mil'el, but, according to the m'sora, both are mil'ra. The m'sora was misinterpreted to mean that hoshia is mil'el while hatzlicha is mil'ra, and that's the way people read it now. Source: an old
mesorah-listserv thread on the topic, in particular one message from that thread, quoting Rabbi Mord'chay Breuer.
As to why the m'sora specifies an ungrammatical reading on this pasuk, I don't know, but such a circumstance is not uncommon.
EDIT: See this picture of the verse as written in the Aleppo Codex which indicates the mil'ra emphasis on each of the two words. (Note also the dagesh in both instances of the word na, which is also discussed in the listserv message linked to above.)