The problem is that there's only so many infrastructural projects that are "shovel ready". Once you've financed those, entirely new projects will only get to the heavy duty building stage in a year or two from now, whereas this stimulus bill is meant to provide an immediate injection of capital into the economy to prevent or at least slow down a rapid slide.
The tax cuts to upper middle class incomes that the Republicans so fervently support have a similar problem. Those people would mostly just save the extra money, greatly diluting its immediate power to stimulate the economy.
Extra money for stuff like food stamps, extra funding for school meals, unemployment benefits, assistance to lower-income Americans who are unable to meet their heating and electricity costs -- all of that, on the other hand, has an almost 1:1 power to stimulate the economy, as it is spent immediately to pay for food and basics. Those all happen to be things the Democrats' Progressive Caucus have put into the bill