"not... " puts the answer and feeling squarely in the 'but' phrase; and even though you add 'solely' to the 'not', the feeling is still strongly with the 'but'; however you want instead to make the 'not solely' either the main feeling or at least the 'not' and 'but' equal in feeling, so you reduce the 'but' into second place of feeling by adding 'also'. Words are as much about painting with feelings and balancing them to the right strength as they are about any facts they convey.
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l3arn3r: Why "also" is necessary?Only because the accepted coordinate conjunction includes that word: 'not only/solely/just...but also'.