Because Catholic permanent deacons made their promises in the context that they were being ordained deacons (and only deacons) for life. The Protestant
ministers (not priests, but ministers) are in a different situation altogether. When they became a Protestant minister (which is, by the way, a ministry not recognized as an ‘ordination’ by the Catholic Church), they made no such promise. So, when they enter the Church, there’s a discernment process, through which they (and the Church) discern whether they’re being called to the priesthood. Catholic deacons, likewise, have already gone through discernment: and the result of that discernment is that they are called to become a permanent deacon.
There aren’t more “benefits” given to priests than to deacons; nor are there more “benefits” given to deacons than to lay people (whether cradle Catholics or converts). A vocation is a
vocation, not a
benefit.