Mercury’s surface temperatures are both extremely hot and cold. Because the planet is so close to the Sun, day temperatures can reach highs of 800°F (430°C). Without an atmosphere to retain that heat at night, temperatures can dip as low as -290°F (-180°C).
I think this is the big reason. It’s also close to the sun, which makes it difficult to land something on it. There’s likely not as much value in doing it just yet, while we can gather data about its composition through other methods. Eventually we might do it, and I imagine it’ll be for resources of some kind
https://www.space.com/41664-mercury-lander-mission-study-proposal.html
I found this article just now but have read it all yet. Seems relevant
Mercury has been devoid of spacecraft companions since NASA’s Messenger mission ended in 2015, and while the next mission bound for the innermost planet launches later this year, it won’t arrive until 2025.
On the last point, here is that new mission and the updated timeline
https://en.m.wikipedia.org/wiki/BepiColombo
On 15 May 2024, ESA reported that a “glitch” prevented the spacecraft’s thrusters from operating at full power during a scheduled manoeuvre on 26 April.[9] On 2 September, ESA reported that to compensate for the reduced available thrust, a revised trajectory had been developed that would add 11 months to the cruise, delaying the expected arrival date from 5 December 2025 to November 2026
It’s expensive and the conditions are harsh.
The daytime side gets hot enough that a rover would be difficult to operate for long. You’d also be getting big swings between daytime hot and nighttime cold, so thermal expansion would probably be annoying.
Then it’s unusually expensive because orbital mechanics make it very difficult to approach the sun. We’re currently all flying sideways with respect to the sun, so if you launch something, it just wants to continue that orbit. In order to get closer, you’d need to shed most of that momentum, which takes a whole bunch of energy since inertia in the vacuum of space just means everything keeps going forever.
Land the rover in the twilight, then have it drive ahead of the sunrise using solar power.
It is the most difficult planet to land upon, but a solar sail could aid in slowing down. Mercury would be excellent for mining and to deliver resources throughout the entire solar system.
The issue is less that it’s the hardest to land on and more that it’s the hardest to get to, to arrive at and orbit. It takes less fuel to get to Pluto than it does Mercury.
Solar sails negate the need for fuel.
Can a solar sail get something from Earth to Mercury?
As a matter of fact, yes! Tacking a sail side to side allows a sailboat to sail upwind in a zig zag pattern. With a solar sail, a spacecraft can tack away from the direction of orbit, slowing down to reduce its escape velocity, and allowing the Sun’s gravity to pull it closer. All using no fuel.
Veritaseum breaks down the principle. https://youtu.be/jyQwgBAaBag