The reasons are many and I doubt that I know them all. However, a straighter exhaust is one. The principle being that the exhasut gas leaves the comustion chamber DOWNWARD and thus the exhaust manifold points down so as not to impede the exhaust gasses. If the engine was not tiped forward, the exhaust would need to bend upward again, which would resist the gas flow. Tipped forward, the exhaust can have a straighter exit.
Secondly, it increases the Airflow to the head; tipped forward, more air strikes the fins.
There is an argument that it also allows more air to 'spill' over onto the exhaust and the rear-'face' of the engine; both of which suffer the highest temperatures.
Another reason was historic. Most 100ccs used to be Rotary Valved. that placed the inlet 'tract' direectly above the drive sprokcet and chain. If the rear sprocket was large, then the chain approached the engine and carb at a steeper angle which, without rotating the engine forward, would have placed the chain against the inlet tract like a chain from a chain saw. Tipped froward, the tract was clear of the chain.
Other may be able to give you more reasons.
The most elegant engine (in my opnion) was a 100cc that was tipped FORWARD so that the piston moved parallel to the ground. This gave PROPER cooling to the exhaust ports. The engine wasn't a success, however!
Ian
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