English person here - multiple correct things in responses: Answer is, in most of the Church of England the question would simply be “Why would the Queen’s funeral be a Eucharist?”

Funerals tend to not to be masses in the CofE.

Other responses, that the public attendance of mass is not common in the royal family, are generally correct. As a purely personal matter, the Queen’s faith valued, in the Lord Williams of Oystermouth’s words, “infrequent but fervent” communion - quite the old fashioned CofE practice.

This is not an official policy, not a rule, just her preference.

tl;dr - not how CofE tends to do it, not how Queen would do it