Yeah, methane is kind of second best (or worse) in many parameters, but most importantly, it's cheap, abundant, and easily and safely storable and transportable, and it does the job.
And while not the reason, also on Venus! Venus seems like a very interesting colonization target - gravity almost like on Earth, and there is a place in Venus atmosphere where temperature is around 30 degrees Celsius and pressure is 1 atmosphere (Earth); and human air is a lifting gas in Venus atmosphere. As a bonus, interaction of Venus atmosphere with the Sun produces a magnetic shield.
Staying in orbit means risk of catastrophic failure on puncture and doesn't provide access to the heat of lower layers of Venus atmosphere that can be used as energy source or carbon source. You also have to think about heat management - big colony means gigantic radiators. And most importantly no gravity and no magnetic shield - these make Venus imho better colonization target than Mars.
On Venus, a puncture doesn't immediately destroy anything - because of equal pressure on each sides of the balloon wall. You have more than enough time to put on a protective coat and fix it.
And building more living space is much easier + you could source the material (carbon) on site.
There are better fuels in terms of Isp and density. They have some downsides such as being corrosive or having highly toxic exhaust.
https://x.com/ToughSf/status/1453391050681327622