I'd say there's two main justifications:
1. Lower Risk - based on 40+ year old proven rockets. NASA can't have failures if it wants to get future funding.
2. Jobs, Jobs, Jobs! - spreading out jobs means politicians are more likely to approve funding
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40 year old rockets proven to kill 17 people and miss every cost and performance metric when they were operated.
Were any of those deaths due to a failing engine though? That's the only part they're really reusing afaik
They are reusing very similar SRBs, which killed one crew and complicates SLS abort modes since SRBs cant be turned off and Orions parachutes will melt if they come into contact with SRB exhaust.
The RS-25 also requires Hydrogen fuel, which in addition to being a terrible first stage fuel because of its low density and heavy tankage (which is why SRBs are required) also has been a causative factor in most of the SLS delays to date.
The RS-25 is viewed as the Ferrari of engines because of its complexity and high ISP (which is only because it uses H2 as fuel). The main thing it has proven us that it’s far too expensive for any useful application. When used on the Shuttle each RS-25 had to be entirely torn down and rebuilt between flights, a process that took months and great expense.
So the SLS takes these enormously expensive engines and burns them up after each use. The only reason RS-25s aren’t long ago put into junkyards is congress trying to favor existing space contractors.
A keralox or metholox first stage would not need super expensive SRBs, would be far easier and cheaper to build. Raptors cost $2M each in prototype form and are expected to cost under $1M in production. Each raptor also provides more thrust than an RS-25, using a far denser and easier to handle fuel.