Why it is that mercury is chosen for thermometers?

why it is that mercury is chosen for thermometers?

Ah this is a fantastic question and goes right to the heart of why we choose different materials for different purposes. You are spot on, mercury (which happens to be my second favourite element) is a pretty poor conductor of heat when you compare it to other metals.   

Thermal conductivity 

We can compare how well materials conduct heat by their ‘Thermal conductivity’, which is often given the symbol ‘k’. Copper for instance has a thermal conductivity of around 380 Watts per metre per degree Kelvin (W/m•K) whereas mercury is just 28.9 W/m•K. Higher values of thermal conductivity generally mean better conductors.  
So if we were looking for a material that conducted heat well then we’d go for copper over mercury every day (or even carbon in the form of a diamond with a reported thermal conductivity in excess of 2000 W/m•K!) However, conducting heat isn’t the only property mercury has in its toolkit and it turns out isn’t as important as you might think to work in a thermometer.. 
Mercury is a liquid at most temperatures we find naturally on the surface of our planet (it freezes to a solid at – 38.9 °C and boils to become a gas at 356.7 °C) it is one of only two elements that are liquid at room temperature (usually defined as 25 °C) the other being bromine.  

Thermal expansion 

So why does it matter for a thermometer that mercury is a liquid? Well thermometers are instruments designed to measure the temperature, and to make that measurement you have to somehow detect or show that change. Most materials expand when they get hotter, there is a great experiment you can do at home to test this:  
Get an ‘empty’ thick-walled plastic or glass bottle (one that won't compress easily, like a tomato ketchup bottle) and stretch a deflated balloon over its opening. Place this bottle into a bowl of hot water (not boiling and be careful if you are using glass – rapid changes in temperature can break some types of glass) and watch what happens to the balloon. As the air inside the bottle heats up, it expands to take up more space – this results in an increase in pressure in the bottle, which should inflate the balloon a little.  
Most materials (but not all!) expand to a greater or lesser extent when they increase in temperature and mercury is no exception. What is happening is the particles (atoms or molecules) of the material vibrate and move more with increasing temperature – increasing the distance between them and therefore expanding the volume the material takes up.  
This is much greater for liquids and gases than solids (although it does happen in solid materials too – remember my warning about glass? It can crack due to the glass thermally expanding unevenly when heated and putting strain on the material). 
So mercury expands in all directions when it gets hotter and contracts (takes up less space) as it cools down, and this is a change we can measure visually with our eyes if we put the mercury in a specially designed thin glass tube. The level of the mercury in the tube therefore shows the temperature.  If we were to use a copper rod instead, we would visibly see nothing change over the 0 – 100 °C range we are often most interested in measuring with thermometers. 
We call this expanding with temperature the ‘coefficient of thermal expansion’ and mercury’s is pretty high, expanding about 0.018 % in volume for every degree of temperature increase. Copper on the other hand only expands around 0.00017 % (this is a simplification and in reality it is a bit more complicated comparing how a solid expands verses a liquid). Early thermometers used alcohol which expands in volume around 0.109 % for every degree of temperature increased but this was often found to be too responsive to be able to use effectively, imagine trying to measure the temperature of something hot but by the time you’ve pulled the thermometer out to have a look, the level of the alcohol is already rapidly decreasing. Mercury is also more useful over a wider range of temperatures than alcohol. 

In conclusion 

We have used mercury in thermometers because it expands in a consistent way when heated, not because of its conductivity. It has been used over other liquids because its rate of expansion is just right for us to be able to make observations in a temperature range we are interested in from a calibrated thermometer. Mercury thermometers are now being banned across the world as mercury is highly toxic to both humans and the environment. Digital alternatives with similar and better ranges exist as well as alcohol thermometers.