Trump wanted a large tax cut. To make the numbers look better (notably the CBO reporting), Congress looked for ways to increase revenues. For whatever reason, they settled on software development, and devised Sec 174 to generate tax on 80% of s/w developer salaries in the first year it went into effect.
Why s/w development? I have seen no indication of the reasons for this; I suspect it was perceived as a "rich field" and thus suitable for this sort of treatment. Also, somewhat more fairly, s/w developers do tend to produce semi-durable assets that in some ways are like capital goods.