The idea behind not being able to vote on accounts with little reputation has a few reasons.
Spambots will abuse the system to create fake posts and use other accounts to vote up questions so their post gets more attention. Due to the sheer volume of users visiting the site per minute, its not doable to moderate this.
Not being able to do something, but learning that with a little bit of contribution to the site, one will be able to actually do more stuff, is a great way to get people to contribute.
The more you do on a site and the more powers you get, the more users are willing to do so.
- Learning the ropes works best if few skills are added at a time. If you have the full set of tools available from the start, you don't learn how to use them well. But if you earn a subset of tools every now and then, you far better learn why you got them, how to use them and even more importantly, how not to use them.
With voting, you give rep for voting up, but you lose rep when voting down. This is important to learn and if you get all the tools, you won't understand the power behind it, but by getting access to them one step at a time, you understand when it is still a good idea to vote down even if it costs you points, and it also helps to make quality posts and answers which is the whole goal of this site.