Jango Fett works for Dooku and Dooku works for Palpatine. Palpatine doesn't really want Padme dead (she is very useful for him, being Anakin's weakness). In fact he does everything to get them together after failed assassination attempt. What Palpatine really needs is that Jedi discover Kamino and Clone Army, in order to proceed with Sith Grand Plan culminating with Order 66.
Kamino was erased from Jedi Archives (in Legends it was Dooku before he left the Order) , to allow cloners some secrecy while creating the Clone Army. But now, this army is completed, and Jedi would have to be led to Kamino, without raising suspicion. And what better way then to create emergency where Jedi like Obi-Wan Kenobi, with lots of connections in underworld, would "unexpectedly" find trail that leads to Kamino. IMHO, Zam Wesell was set up to fail and that is the real reason Fett didn't try to kill Padme by himself - he would succeed, but then Clone Army would not be discovered. In worse, Military Creation Act would pass and Republic would have regular (non-clone) army which would be much harder to control .
Out-of-universe, Lucas is simply not very good in storytelling. He needed some reason to bring main characters (Anakin and Padme) back together and in spot light, but he also needed some reason to explain discovery of Kamino. Therefore, he invented this rather convoluted and illogical story with outsourced assassination contract and strange dart.