The same reason you shouldn't put next week's gas money into fusion energy research. Some problems are more immediate than others. We definitely do need to put more money into geo-engineering research but in the short term he have to cut fossil carbon use, or its going to hurt us in a way that will make it harder to do that geo-engineering research.
Except there is no "short term" here. Anything we do to reduce CO2 levels isn't going to make a measurable difference any time soon. It's going to take decades - at the very least - before we see any meaningful effect. More likely we'll never see any effect from it at all, given the pace at which India and China are making up for our reductions. So no matter how you look at it, it makes far more sense to look for other ways to control climate.
Launching sulphur into the atmosphere or other plans of reducing sunlight (I've also heard of using large clouds of particles in space) are just a band-aid that only addresses one aspect of CO2's effects. It does nothing about ocean acidification for starters, and will have many side-effects that reducing atmospheric CO2 wouldn't.
And reducing CO2 is just a bandaid solution because it doesn't address the natural cycles of warming and cooling. I would rather have a "bandaid" which covers many contingencies than one which covers only one specific hole.
Right now we sequester, if the planet starts to get too cold later we can burn the sequestered carbon to raise the temperature again.
That might make sense if you could sequester it in a "burnable" form. We can't do that without consuming more energy than we produced in the fist place. For that plan to be at all feasible we would have to develop a way to produce essentially limitless extremely cheap energy, at which point sequestration becomes somewhat of a moot issue anyway.
It's also a crappy way of going about it - the response curve to carbon is just too slow. I don't think anyone wants to live through 40 years of cooling, hoping that the trend will reverse before the temperature drops low enough for crop failures to cause mass starvation.
Lastly, think about the scale of what you're proposing. Sequester all the carbon which our entire species has produced over the last 100 years, then burn it all again as quickly as possible if an ice-age comes along. Do you have any idea how massive of an undertaking just the first part would be? You may as well scrap all other industry - this project will keep us all employed for decades to come. Better find lots of money trees to fund it all.
