Why do diesel engines have,,,

The IH start on gas diesel and JD and Cat with pony motors would be the only diesels you could start by hand, since you start the gas engine first.

Rather the reason for lower compression on the early diesels was technology. The stresses of high compression diesel operation were simply beyond the limits of the materials, lubricants, etc. of the time.

Diesel engines use the heat of compression to lite the fuel/air mix. The older diesels didn't have high enough compression ratios to lite the mix at cranking speeds or cold temperatures.

Newer diesels have higher compression ratios and this contributes to the better cold starting.

ALSO....the faster a cold diesel is cranked, the colder it will start unaided. Modern electric cranking systems will spin a cold motor much faster than the systems of 60+ years ago. This improves cold start capability too.

Other methods have been used to assist indirect-injected diesels: Intake manifold heater (some Case and Oliver engines). Pony motor - provides faster cranking speeds and pre-heats the diesel block and intake manifold (used by Cat and Deere). And of course the IHC "start on gas" diesel (starts as a low compression gas engine then switches to diesel after it warms a bit)

Most modern diesels however are direct injected i.e. they have no pre-combustion chamber. Direct injection's advantage is it's greater thermodynamic efficiency over an indirect injected diesel. However there is not enough space in a direct injected combustion chamber to fit a glow plug. Offsetting this, a direct injected diesel is inherently somewhat better at cold starting than an indirect diesel. However even a direct injected engine will not start below a certain temperature. Devices such as intake manifold heaters (to heat the incoming combustion air) or ether injection are employed to assist cold starting.