Because we have better ones, that are less expensive, in many (but not all, due to excessive design rigidity in some cases) cases cheaper to adapt to change, and provide more value. That benefit is significantly limited if you bear the cost of maintaining (across requirement changes) the old processes (or analogs to them reliant on similar infrastructure) as well as the new processes, especially if you have to maintain the infrastructure (including the human infrastructure -- e.g., for the IRS, of people trained to process tax returns by hand) they rely on.

> However, the IRS maintaining the capability of using old, simpler methods should be retained because it serves as a backup for (hopefully rare) situations where the primary infrastructure fails.

Certainly, functions that are short-term critical need some fallback. Functions that aren't, it may be more efficient to devote resources to restoring the primary infrastructure rather than burning them executing inefficient backup processes in the absence of primary infrastructure.