Because communism rests on abolishing property. Practically, you can't abolish ownership without a de-facto authoritarian police state. That follows naturally from the idea of "seizing the means of production" - you can't do that if the capitalist next door pays more money than your cooperatively owned factory where nobody really owns anything, because people will just choose to sell their labour to the highest bidder. So the only option is to make owning things illegal.
In the USSR this is exactly how things worked - you absolutely could not own .. stuff. I mean, you could buy shoes and those were your own shoes, but you couldn't buy, say, a factory. You couldn't buy land to build a factory. Heck, you couldn't buy land, period. You could not choose to live off the land, because all the land belongs to everyone^Wthe state. You worked where you were told (based on education), when you were told, the salary for your position was set in stone, and the fruits of your labour were divided fairly - half to the top members of the party, half again to the middle managers, half again to good friends, and the rest to all the other members of the party (everyone is a member of the party, not being a member of the party is illegal).
This form of government can work and if you remove the corruption element and actually manage it intelligently, it can work incredibly well, and I'll bet you a million dollars there are plenty of people who will love a form of government that removes all choice from their lives.
That said, the form of government, that is probably most close to what Marx envisioned, is maybe direct democracy, with corporations that don't have CEOs, but where decisions are taken again via direct voting (N.B. I might be talking out of my ass here). Those kinds of corporations do exist, but the practice hasn't caught on widely.
When you boil it down, we don't want people to be serfs. The ideal is that everyone captures the full value of their labour, without trampling over the rights of others, and that everyone has the opportunity to enjoy a high standard of living. Getting there requires a really careful balancing act, where you give the people in government just the right amount of power, so that factory owners &c. can't oppress their workers, while at the same time it's not so much as to swing back to an authoritarian system with monarchs and serfs. Currently we call this "democracy" and the economic system "capitalism", but I would like people to not think in magical words and focus on the actual essence of problems and solutions.