Why won't Medicare pay for Lexapro?

Lexapro is a newer form of an older drug (Celexa). The bonus of Lexapro is that it has less side effects than the original drug because it is molecularly different (in essence, the parts of the original chemicals in Celexa that were unnecessary and which cause many side effects were removed, so it's a purer form of Celexa in a sense). However, many insurance companies will only pay for the older drug because it works in the same way and it's cheaper. Insurance companies apparently don't care about side effects. ---- There are many Medicare Part D companies. Each is different. Medicare does not pay for certain classes of anti-anxiety drugs.