To be more precise, there are 3 things that must be met for a vote to be considered acceptable, digital or not:

* it must be anonymous, ie nobody but you should know you you voted for

* it must keep integrity, ie nobody can change your vote

* it must be verifiable by all participants

Paper vote makes it easy to meet all criterions: just keep an eye on the ballot and you will be relatively sure that everything is fine. On the other hand it is impossible to have all those conditions with a digital vote, if only because digital anything isn't easily verifiable by all participants.

Implementing anonymous, distributed, publically auditable voting is quite simple. Implementing it in such a way that you can verify that your vote was counted correctly, but not be able to present it to a coercive third party to as a receipt to verify you voted for their preferred candidate, is hard.