To build on what Lapey said, I think some of these points become clearer if we look at how the diaconate is something other than a religious vocation (in the formal sense of ‘religious’).
After all, plenty of men are called to a celibate religious life but are not called to be priests, these are the religious brothers, who contribute so much to the life and witness of the Church. But brothers’ vocations are almost always some form of Christian service (teaching, nursing, ministering to the homeless, etc.) which is hardly incompatible with the diaconate, so why are they not all ordained deacons?
Being a deacon is neither an occupational vocation (though it can be linked to one), nor is it membership in a religious order. It’s a sacrament: the sacrament of Holy Orders. As per the catechism and Vatican II, this sacrament is found in its fullness among Bishops. Therefore both priests and deacons participate in this sacrament in a more limited way. In that sense, it’s not wrong to think of priests (presbyters) as “bishops light” and deacons as “priests light” (sacerdotes).
Of course, priests (presbyters) and deacons’ participation in Holy Orders is of a fundamentally different nature–a priest is essentially a local stand-in for the bishop, who can’t be everywhere at once, and the deacon is there to *assist *the bishop (and by extension, his priests). A deacon is not a stand-in for a priest.
Even so, I think the fact that it is a sacrament, to which one is called by God through the Church, means that (like it or not), a priest who is called to be a bishop, unless there’s a valid reason not to, is expected to answer that call even if (as is often the case) he has no desire to be a bishop. (Indeed, the Holy Father’s order explicitly forbids any aspirations to the episcopate among its members!)
And so (finally getting around to the original question), I think that a deacon who finds himself called to the priesthood by his bishop will have a duty to answer that call, unless there is a valid reason not to. (In the Latin Church, of course, being married is an valid reason not to be called to the priesthood.)
Therefore, given the shortage of priests in the world, it should not be surprising at all that celibate deacons are quite rare–most wise bishops *will *call upon those deacons they can in good conscience call to the priesthood, and *their *consciences will oblige them to accept the calling. I’m sure there are many holy bishops who, if it had been up to them, would have been happy to remain deacons their whole lives (as indeed they do, but I mean without being ordained further). But it is not up to them–or up to anyone on their own–to decide whether and how they will be ordained.
Ordination marks a man forever for service to the people of God; it’s a different category of thing from (but not incompatible with) the pursuit of sanctification under the charism of a religious order (be it as a consecrated brother or as a married tertiary). I don’t really agree with the original poster’s premise, then, that “The main role of the deacon, is to reach out to the downtrodden, the sick, the poor, etc. and help them live better lives and bring the Gospel of Christ to them.” That certainly is *an *important role deacons play, but liturgical functions, preaching, and assisting the bishop are given just as much emphasis in the catechism.
It is the task of deacons to assist the bishop and priests in the celebration of the divine mysteries, above all the Eucharist, in the distribution of Holy Communion, in assisting at and blessing marriages, in the proclamation of the Gospel and preaching, in presiding over funerals, and in dedicating themselves to the various ministries of charity.
The main role of the deacon, I would say, is to be a cleric, and to manifest this clerical role and presence in the particular ways that belong to the order of deacons. This is what sets them apart from religious brothers, sisters, and baptised Christians generally, all of whom, like deacons, can and should, insofar as God enables them, ‘reach out to the downtrodden, the sick, the poor, etc. and help them live better lives and bring the Gospel of Christ to them’.