Because this is not what the Polish sentence says. "Chcieliśmy wam pomóc: means exactly that we wanted to be helpful in whatever you (plural) are doing.
Your version would probably be "Chcieliśmy, żebyście/byście/abyście (nam) pomogli"
I put "nam" in brackets because your sentence doesn't specify if it's 'to help us', I just considered it the most probable option.
Wiktionary entry for 'byś', the others work in the same way in conjugation.