They were so mobile,had good firepower and everything was ''perfect'',

They were far from perfect. The early British marks of "Tank" (FYI, they were the Tanks, Mks 1-9, rather than "Mark Tanks") were slow, of questionable reliability, cramped, hot, extremely loud and generally uncomfortable and potentially dangerous for their crews. Already by the end of WW1, a number of alternative light tanks were in operation, such as the British Medium A "Whippet" and the French FT-17. These offered a number of improvements, being designed to be faster and more mobile, requiring fewer crew and, in the case of the FT-17, mounting their armament in a more convenient and effective rotating turret. The 'Whippet' proved particularly effective during the Amiens offensive, where its speed allowed it to break through ahead of the heavier British tanks and over-run German artillery positions in the rear. The FT-17 was the far more revolutionary vehicle: faster and more manoeuvrable and with a crew of only two, it was able to carry either machine gun or cannon, and to deploy them far more effectively by use of its fully rotating turret. Its rear-mounted engine in a separate compartment and lower tracks gave it much more effective mobility than heavy tanks, while also being safer and more comfortable for the crew.

Tank development lagged considerably following the end of the war in 1918. After all, the first Tank had been specifically designed for the trench warfare conditions of the Western Front and the open warfare of the Hundred Days showed the benefit of smaller, faster vehicles. In post-war disarmament, many nations shied away from the expense of heavy armoured divisions. Britain was one of the few nations that did focus on modernising and mechanising its forces, but it focused its efforts largely on the development of armoured cars rather than tanks, which were highly mobile, cheaper, and crucially much better suited for the colonial policing duties that previously had been a major task of mounted cavalry.

This is a very interesting talk by the curator of The Tank Museum, Bovington on post-WW1 tank development and the influences upon it that I hope you might find useful.