1) Compression Ratio: Diesel engines are characterized by high compression ratio. The compression ratio of typical diesel engines are in the range of 15:1 to 20:1. Higher compression ratio means higher pressure in the cylinder. The reason for higher compression ratio of diesel engines are that diesel as a fuel typically ignites at a temperature range of 180-330 degrees centigrade (autoignition point) and in diesel engines the diesel fuel is ignited by subjecting air to a high pressure (due to which the temperature of the air increases to 430-650 degrees centigrade) and injecting fuel into the hot compressed air in the cylinder.
2) Sturdy engine block required: Due to high compression ratios, which in turn means higher pressure the cylinder is subjected to, the engine block must be made stronger to withstand it. Hence, the diesel engines are more expensive than their petrol counterpart.
3) Weight: Due to the above reasons, the weight of the engine (in turn, the weight of the bike) will increase.
4) Ancillaries: Diesel engines require fuel pump, glow plugs, etc which all put together will increase the overall cost of the bike.
And diesel engines are chiefly used where torque (pulling power) is a preferred rather than outright power (trucks, buses etc).
You only see diesel engines in the scale that you do now-a-days because of the advent of vastly higher injection pressures due to the Common rail and Unit injectortechnologies. Without very high injection pressure enabling well burning fine fuel mist, diesel engines are slothful and emit a lot of smoke. I don't know any manufacturers that have tried implementing this in a two-wheeler.
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