Well the main reason I believe that the Fw 200 was not used as heavy bomber is because the aircraft was structurally not strong eneogh. The aircraft was designed as a commercial aircraft before the war and was converted for military uses.
The military version of the aircraft had a tendency of breaking apart when it had a heavy load.
There's a book called the "300 best aircraft of WWII" it has a lot of info on the condor.
THis is from the book ( I am not making this up)
"The Condor is best remebered as the long rang-reconnaissance aeroplane searched for Allied convoys in the North Atlantic during WWII and then either attacked them directly with bombs or missiles or vectored in packs of German U-BOats. The type was designed as a transatlantic airliner, however, and first flew in this form during JUly, 1937. THe first of 259 Fw200C military aircraft entered service in September 1939. A few of these aircraft were used as VIP transport, but the majority were used as long range reconnaissance bombers in seven subvariants, some of which spawned their own subvariants with different armament fits, radar fits and provision for missiles carriage and guidance, as well as stripped down forms for special transport task.
Type: six seat maritime reconnaissance bomber.
Armament of a 200C-1: one 20 mm trainable cannonin foward ventral gondola, one 13 mm trainable machine gun in rear dorsal position, one 13 mm trainable machine gun in beam positions, one 7.92 machine gun in foward dorsal turret, plus a bomb load of 2100kg (4630lbs)
As others have stated the origins of the Fw 200 was that of a civilian airliner, however it was the Japanes that chanaged that!
"The Japanes, always interested in aeronautical developments abroad, evinced a marked interest in the Condor, and on November 28, 1938, the Fw 200V1 took-off for Tokyo, stopping at Basta, Karachi and Hanoi for refuelling, and arriving at the Japanese capital in slightly less than 48 hours of which 42 hrs. 18 min. were flying time.
Apart from placing an order for five Fw 200B transports .... impressed by the Conder's long range capabilities, were interested in an adaptation of the Condor for the long-range maritime reconnaissance-bomber.
To meet Japanese requirements, the Focke-Wulf company began work on adapting the Fw 200B for maritime reconnaissance duties early in 1939, and the result was the Fw 200V10, destined to become the forerunner of the Fw 200C series of commerce raiders"
Source: Famous Bombers of the Second World War by William Green