[Romani]

Tekko, that's a frequently asked question and the reasons are many, I will try to summarize them, I have numbered them but in fact they were interrelated.

1) Insufficient power of available engines: Up until the Hispano Suiza V-8 engine appeared, the Allies were stuck with light rotary engines , performance was severely affected when the 50 kilos of a second machine gun and ammo were added. The Albatros was the first fighter that could carry two machine guns without performance being severely affected.

2) Unreliability of early synchronizer gears: This led the British specially to continue relying to a great degree on the Lewis on an overwing mounting, wich also was lighter.

3) Lack of foresight: A single MG was deemed sufficient at the time the first allied fighters appeared (1916) and carried on the designs that were on the drawing board and entered service the next year, such as the SE5. No thought was given to the possibility of adding another gun later on, the SE5 did have enough horsepower to carry twin synch guns, but the way the nose was designed there was no room to add a second gun and ammo. At any rate, a Vickers plus Lewis was deemed good enough.

The engineering margin was narrow anyway. The SPAD VII might have had enough HP for twin MGs, but it would have needed likely a larger plane so you will have end up with something no better or worse even, than an Albatros.
The SPAD XIII was a larger airplane with a more powerful engine and even so the handling characteristics were worse than the VII.

4) Shortage of Vickers machine guns:

The only Allied machine gun suitable for synchronizing was the Vickers and the British had not only to supply their army and aviation, but also the French aviation. This was too another reason for the British preference for the Lewis, you could build 3 Lewis for the time and effort to make a single Vickers.