Your question should be 'Why is it correct?'. Don't forget that the word order for questions is (1) Question word (2) Verb (3) Subject. But, that aside, it's a good question.
And the answer is this:
We use 'the' precisely BECAUSE we are speaking in general. For example:
The elephant is the world's largest land mammal.
Mankind's greatest invention is the wheel.
In these cases, the use of the definite article 'the' before a singular countable noun indicates that you are speaking in general terms about this topic.
As for your alternative examples:
No, you can't say 'Bicycle is becoming very popular.' This is ungrammatical and sounds like pidgin English. In correct English, you have to have an article, or another determiner, before a singular countable noun. The only way this sentence could make sense is if 'Bicycle' were a proper name, such as the name of a restaurant or a trademark, for example.
Yes, you could certainly say 'Bicycles are becoming very popular.' The only error is that you forgot to put a capital letter at the beginning of your sentence. This has a similar meaning to the original sentence, although it isn't quite the same.