padawanlost:

Due to the combination of his being really an examplary Jedi (whatever
one makes of that), sympathies/nepotism on the part of the elder
Councillors, and the fact that they needed him to have authority over
Anakin while he continues to babysit him on their behalf (it sounds ugly
put like that, but since Mace flat out confirms that Anakin never
gained his trust, it seems a logical conclusion for the
Anakin&Obi-wan team-up that happened after the former was raised to
the level of a knight)
       
   

I’ve
explained how the Jedi ranks/titles work in detail here,
if you are interest. To answer your question, yes, Obi-wan was knighted right
after killing Maul but there’s no such a thing as a “low rank”.  Being knighted was a great honor, the dream of
all padawans. It also meant you are no longer an apprentice and could take all
your rights as a full Jedi.

• Jedi Initiate: the
  youngest members of the Jedi order. They achive their rank when they are
  old enough for individual instruction (2 years old for humans).
• Jedi Padawan: from adolescence (12 years old for humans) until you become a Knight or are reassigned to the Service Corps
• Jedi Knight: If
  you successfully pass your Trails.
• Jedi Master: highest formal recognition one can achieve in the Jedi Order.
• Jedi GrandMaster: is
  a title bestowed upon the greatest of all Masters. Only one single Jedi
  can be appointed Grand Master at any one time.

Most Jedi lived and died as knights. Only very few were ever given the
title of Master. Padawans called their teachers masters, but that was an
honorific, not their rank. In Obi-wan’s case, this is what happened:

TPM

• Rank = Padawan
• Honorific = Padawan

AOTC

• Rank = Jedi Knight
• Honorific = Jedi or Master (to Anakin, padawans and initiates).

TCW/ ROTS

• Rank = Jedi Master.
• Honorific = Master.  

Obi-wan was knighted for killing Maul (proving his skill) and because
his master was dead and the council decided that instead of finding him a new
master, Obo-wan was ready to be a knighted (and eligible to train Anakin). Obi-wan,
as a Jedi Knight, trained his padawan Anakin Skywalker until after the Battle
of Geonosis. For his role in Geonosis and for successfully training Anakin, he
was rewarded the rank of Master and invited to join the High Council.

Obi-wan was obtained his rank of Knight and Master because he was a
really good Jedi (skilled + talented + obedient). He managed to train the
untrainable Anakin Skywalker despite everyone having no faith he could. He
killed a Sith Lord. He was involved in many battles, conflicts, diplomatic
missions, etc. when we talk about the Council’s nepotism and bias, we are not
saying Obi-wan didn’t deserve recognition. We are saying the Council’s method
of recognizing Jedi were arbitrary and biased against certain types of the Jedi
(females, independent thinkers and anyone they might consider “subversive”).