What is the main reason (attacks) the appearance of the protocol SSL version 2.0 to replace version 3.0?
RFC 6176 lists four reasons why SSL 2.0 must not be used, in its section 2:
However not all reasons are equivalent in gravity. The main security issue with SSL 2.0 is that it has no notion of verified closure: the SSL 2.0 connection ends when the underlying TCP connection is closed. An active attacker can force that, and neither client nor server can know whether the closure is really what the peer wanted. This is a problem when the protocol within the SSL tunnel uses the closure in a semantic way; e.g., with HTTP when there is no
The three other reasons are less imperious, and can be debated:
Apart from that, SSL 3.0 is more extensible and offers extra features:
Something tells me you didn't put much research effort into this one.
Let me quote part of a distantly related Crypto.SE question by @D.W.:
Can't add much to that one since it wraps it up rather nicely while skipping the need to bluntly list all individual v2 problems that were fixed by v3. Comparing both protocols will enable you to learn about the fixed issues yourself.
In fact, any search engine will enable you to find references to both protocols, as well as a truckload of papers related to discovered security weaknesses that v2 incorporates. Also, the answers to the linked question contain large chunks of usefull, related information you might want to check out.