Why diesel engine is so hard to start ?

You may have a couple faulty glow plugs that preheat the air inside the cylinder to make the fuel burn correctly when starting when it is cold so the engine struggles to fire up.

Diesel fuel is a kind of oil, and it gets thicker in cold weather. In very cold weather it becomes like jelly, so truck drivers don't turn off their engines when it is very cold.

Bad glow plug timer preheat circuit power or no ether left in the auto ether intake injector primer ether aerosol can.

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Diesel does not get a spark like gasoline. It combusts on good injection spray fog pattern and tight (high) cylinder compression. The older the engine, the lower that compression gets, the less foggy the injection spray gets. Other answers rightly point out that a block heater and glow plugs are a big help in cold weather, but as others have said, modern diesel designs are so tight compression-wise they will start on the first go.
Compression test. Injector spray test. That's where to start.

At about 10 deg F, summer diesel gels. Diesel is longer carbon chains that is not as volatile as gasoline, so it does not convert to the gaseous state as easily as gasoline which becomes a problem at lower temperatures. I used to be leery of diesel engines when it was cold, but there are numerous methods (ether injection, glow plugs, grid heaters) that let them start easily as long as the fuel can flow.

With it's very high compression ratio,a diesel requires a very strong starter motor and especially a healthy and high capacity battery or batteries to crank it quickly enough to start and run.

Diesel fuel is an oil that when compressed quick ignites/fires and sends the piston moving. In cold weather it s thicker. Using fuel additives in the winter and making sure the fuel filter is clean helps greatly. Using a block heater is also a good idea for over night parking.

Hi on most modern diesel engine glow plugs are fitted these pre-heat the cylinders so it is no different to a gasoline engine on cars like Toyota's.

Diesel has a lower flash point than gasoline.