Mikhail Gorbachev met with opposition in the Soviet Union largely because he wanted to implement major changes. People who want to change systems in radical ways are often met with opposition. He also met with opposition because his reforms did not bring about any sort of immediate prosperity.
When Mikhail Gorbachev came to power in 1985, he did not want to bring down the communist system. However, he did want to reform it in very important ways. He wanted to open the system up both economically (perestroika) and in terms of political rights (glasnost). At that point, the communist system had been in place for almost 70 years. Anyone who wanted to change it in radical ways was bound to encounter some degree of opposition.
In addition, Gorbachev’s reforms did not really work all that well in the short term. The new political openness led to political conflicts. There was much more discord within the country with (among other things) some parts of the country wanting to break away and become independent. The new economic system led to widespread problems as well.
Thus, Gorbachev encountered opposition both because his reforms were radical and because they did not achieve immediate success.