Natural monopolies have high fixed costs and low marginal costs. Software has high fixed costs since you have to pay expensive developers to write the code first, and then low marginal costs to run it on relatively cheap servers. That fits the definition perfectly.
It's just that the market for cloud computing, mobile, search... Are so large they can support multiple companies so they tend to actually be natural oligopolies.
While natural monopolies may often (or even always?) have high fixed costs and low marginal costs, it does not at all follow that any industry with high fixed and low marginal costs lend themselves to natural monopolies. And there's no reason why they need to be oligopolies, either.
That's just a function of many founders' desire to flip their company for personal profit rather than building a sustainable long-term business. And a function of big-company executives wanting to further enrich themselves at the expense of their employees during big-co mergers.
It's just that the market for cloud computing, mobile, search... Are so large they can support multiple companies so they tend to actually be natural oligopolies.