Because the Jedi council is massively overconfident in the abilities of individual Jedi and massively underestimates the power of the dark side of the Force.
Yoda lifted one skinny arm, pointing at Qui-Gon. “With this Naboo
Queen, you must stay, Qui-Gon. Protect her, you must.”
The others murmured their approval, evidencing the confidence they
felt in the Jedi Master’s abilities. Still Qui-Gon said nothing.
Star Wars Episode I: The Phantom Menace - Official Novelisation
They're also concerned about seeming to take sides in a local conflict between the Naboo and the Trade Federation. Qui-Gon (and his apprentice) are still, arguably, acting on the Chancellor's orders as his personal ambassadors. Adding additional Jedi would require additional approval.
MACE WINDU: Protect the Queen, but do not intercede if it comes to war until we have the Senate's approval.
Star Wars: The Phantom Menace - Original Screenplay