* X is available more cheaply from a third party supplier (who presumably has achieved some kinds of economies of scale) than our fully-loaded cost to deliver the same
* X is a commodity product where we do not feel we can differentiate ourselves by doing it in-house
* X is ancillary, but important, to our core business and we lack the expertise to do it well, so we will buy in rather than build that expertise
Not many hotels are bothering to drill their own wells these days, bearing in mind that water is available from utility suppliers - why would room cleaning service be immune from the same forces?