because the wealth is already taxed - how else did you acquire that wealth without paying taxes first?
To tax wealth is to prevent accumulation of wealth, and i think that has bad, unintended consequences. On paper it sounds good, but like all things, this hurts some group of people more than others, and just because that group being hurt isn't "your" group of people, it doesn't mean they deserve it.
Taxing income (of which capital gains is one type) is fine - adjusting the brackets is fine too. But a lot of times, people come into the more-taxation side assuming that more taxation means better society - but i can tell you it isn't true. Better politicians comes first.