On the surface, of course there is no reason a plan would not want a zero-expense fund S&P 500 Index fund. But your question was is there any reason a plan would not want this zero-expense fund? This is another way of asking, cynically, what's the catch?
From an AI point of view, "Essentially, the "free" thing is being offered in exchange for something valuable to the provider, usually your personal data or attention." Examples are free checking accounts, free shipping, free breakfast with an overnight stay... i.e., something that builds a recurring relationship.
Empower is offering this as an "Institutional Separate Accounts" which their information says are "(also known as insurance company separate accounts are an insurance company version of a collective investment trust (CIT). Like a CIT, institutional separate accounts pool assets from more than one retirement plan to achieve economies of scale and pricing." https://docs.empower.com/empower-investments/pdf/isa/Institutional-Separate-Account-Platform-Brochure.pdf and https://docs.empower.com/empower-investments/pdf/isa/Institutional-Separate-Account-Platform-Brochure.pdf
With this offering, Empower is one-upping Vanguard in a proverbial "race to the bottom" on plan administration and investment fund expenses.
I applaud Empower for recognizing that investment fund fees are an irritant for many plans, and for coming up with a creative solution bolstered by great marketing.
Plan fiduciaries and their advisors still need to do their due diligence and this includes considering the totality of the relationship. If their conclusion is Empower offers the best services for their plan, they should give this fund serious consideration.