Mainly because Brewster were not a very good company to deal with, their factory was very chaotic and not able to produce enough planes. To make matters worse Brewster reps sold planes to just about every foreign power they could which annoyed the Navy brass no end.
The early Wildcat was considered an inferior plane to the early Buffalo, but then the Navy started specifying more armour etc, which meant more weight. This meant adding powerful engines, which were in turn heavier. Grumman were able to update their design to cope with this better than Brewster. The Buffalo suffered from landing gear failures as its weight increased where the Wildcat didn't. The final nail in its coffin was that the Buffalo's fuel tanks were integral to the wing structure, rather like the Betty bomber's. This made it much harder to add self sealing fuel tanks than to the Wildcat.
The early lightweight Brewster we have is a more maneuverable plane than the heavier F4F-4 which has more guns, armour and the weight of a folding wing mechanism to lug around. The earlier F4F-3 would do better, as it was a lighter with the same power.
Now that it's middle of the week instead the end of one I'll add a couple things...
As Greebo explained Brewster Corp. wasn't exactly a well managed company. Brewster Corp's separate selling organization called Miranda Brothers was a source of problems. They had been found guilty of illegal arms trade in the spring of '40 although that particular incident wasn't connected to Brewster Corp. As Greebo largely said, this selling organization made the Co. of the Brewster Corp. sign deals which they didn't have the production capacity for. As the orders and the company grew fast they took on labor force which was bit on the shady side and due to this even sabotages occurred. In one of these incidents F2A-2's arrestor hooks had been deliberately weakened. I'm sure you can imagine that something like that didn't exactly add Brewster's points in the eyes of the Navy.
All the above happened largely
after the Brewster which AH was delivered to Finland.
Then there were the technical problems which made F4F better suited Naval fighter:
- Brewster's landing gear didn't withstand carrier use well. They tended to collapse quite easily and the fact that the take-off weight kept creeping up with the later variants didn't exactly help either. This wasn't a simiilar problem when operating from land bases. There were couple gear collapses in Finnish use but not many.
- The wing was a single piece unit with a single continuous spar. When damaged it was very hard and slow to repair and it really could not have been made foldable without more or less complete redesign.
- Those self sealing tanks which already have been mentioned.
- F4F was more rugged airframe and based on my experience that is the case in AH. Again, only my subjective view regarding the matter. If someone doubts it, you can test it.
- Twin wasp generally was more favored as the fighter engine over the Cyclone in the US military circles.
So next time some of you wonders why a certain plane was replaced by another one in certain country's certain military service branch, try to bare in mind that in real war and life there are few more factors involved than which can turn inside which in an 2012 air combat simulator's main arena.