Because the average price for an Electric Vehicle is almost $60k right now. A $7500 tax credit doesn't really do much to make it more affordable. (Especially since it is a tax credit, not a rebate. So you still have to pay the full price upfront. And chances of getting the full $7500 credit are pretty slim with all of the fine print.) And there's still an average 8 month waiting list to actually get an electric car these days so, if the tax credit makes them more popular, it's going to take even longer since they can't keep up with demand as it is, so increasing it will only make it worse.

I don't need to be currently shopping for an item to point out that a promotion for that item has some drawbacks or isn't really that good of a deal.