1. There was a lot of
competitive selection pressure because nothing was complex enough to have any defence against anything else eating it: that is, complex enough to have any homeostatic abilities to continue to survive when a neighbouring organism dumps metabolic waste products into the environment that make it less hospitable; or complex enough to survive when any neighbouring organism gets close enough to just start using its constituent proteins as reactants in its own metabolism.
When you haven't yet evolved adaptations like "strong cell walls" or "the ability to recognize and excrete substances antithetical to your metabolic processes" or "the ability to store metabolic energy for times of resource scarcity", you have to spend a lot more of your time as an organism either moving away from danger, or reproducing away from danger.
And, in environments that cycle between resource-rich and resource-poor, the "reproducing away from danger" strategy is highly preferred: you can always shoot off a germ cell specialized to survive under such resource-poor conditions—i.e. a spore—until it enters a richer/safer environment; and so, when you're in a rich/safe environment, it pays off to have another morphogenic stage specialized toward "thriving" rather than "surviving." (If you're wondering why you'd do this rather than just "turn into" a spore, and then "turn back into" an adult organism, like some bacteria essentially do when dried out—well, that turns out to require more genetic code, especially when you're already doing mitosis anyway.)
2. Things that didn't have robust reproduction mechanisms couldn't evolve these other complex adaptations (cell walls, homeostasis, etc.), because every reproductive step would introduce too many errors for the adaptive "signal" to survive well-enough to spread through the population.
Before any other adaptation, the most important and urgent adaptation an organism that relies upon reproduction can evolve is to increase the robustness of its copying logic to the point that other traits can be constructed cheaply, through focused, linear adaptation rather than random chance.
Or: the probability of any random complex adaptation evolving without DNA is very low. That includes DNA itself, of course. But the probability of any random complex adaptation evolving given DNA is much higher. So you'd expect that, of all the evolutionary lines that survived, they all ended up adapting to use DNA first—because the ones that did some other adaptation first would still have to also evolve DNA [through random chance] to preserve that adaptation for any useful number of generations. Organisms with DNA will strictly outcompete those without, because DNA makes you able to get to any other adaptation much faster—and keep it once you have it, and build on it with further adaptations, until you've got a complex feature.