The platform already has a steep learning curve for new users. Introducing the voting slider after they have gotten used to the basics is part of it.
Also there is a limit in how small of votes you can cast. If you got it right away you would get errors if you tried to vote below a certain percentage. This is also not good for the user experience.
It is a little frustrating how many "oh by the way" things there are to discover. I, for one, would have appreciated a typed out list of the math and timing rules. .... oh by the way, there's this 30 minute weird window on upvoting for curating $, and oh by the way, you're not really in control of how your votes are distributed, and oh by the way ... there's rumors of other time limits too.
Has anyone thought about how rough it is and how discouraging it is and how shady it feels to self-discover these hidden factoids? I'm rather certain, there's more I haven't discovered yet.
And y'all are worried about trusting us with a slider that could keep us from voting ourselves into oblivion?
Change the error messages into instruction messages. Restrict the range of the slider for newbies, but please don't fail to equip the most active voters on the platform, those who are freshly fascinated.
Have you checked out the FAQ? A lot of that stuff is covered there.
The impact on not having the slider is not as horrible as you make it sound. If you vote a lot, the system will automatically start voting with less weight, since your voting power will be lower. It is the same effect as using the slider, just without as much choice on which posts/comments it gets used on. When you have so little SP anyway, the effects of this are so minimal that most users will not notice.
The decision was made that it is not worth the extra complexity to new users, for the benefit they gain.