Making it plain

Dan Phillips begins what looks like it will become a fascinating new series over at Pyromaniacs, in which he calls upon Calvinists to explain stuff.  He says:

I’m going to introduce a few such popular Calvinist positions, and open the meta for advocates of those positions to explain the rationale in under two hundred words. Speak in words any Christian can understand. Do not preach to the choir. Be pointed, plain, precise, helpful.

First up, in fewer than 200 words, explain why Calvinists reject altar calls.  I look forward to the comments, which have yet to start rolling in.

Apart from specific issues, it is a good discipline, worthwhile not just for doctrinal clarification (legitimate shibboleths?) but also for the gospel itself.