This speech explains why. It's a very important read. I bolded a few sections but it is otherwise un-redacted.

TEXT OF SPEECH – February 24, 2022 (emphases, mine)

Citizens of Russia, friends,

I consider it necessary today to speak again about the tragic events in Donbass and the key aspects of ensuring the security of Russia.

I will begin with what I said in my address on February 21, 2022. I spoke about our biggest concerns and worries, and about the fundamental threats which irresponsible Western politicians created for Russia consistently, rudely and unceremoniously from year to year. I am referring to the eastward expansion of NATO, which is moving its military infrastructure ever closer to the Russian border.

It is a fact that over the past 30 years we have been patiently trying to come to an agreement with the leading NATO countries regarding the principles of equal and indivisible security in Europe. In response to our proposals, we invariably faced either cynical deception and lies or attempts at pressure and blackmail, while the North Atlantic alliance continued to expand despite our protests and concerns. Its military machine is moving and, as I said, is approaching our very border.