The number of tenured/tenure track faculty positions in the US has been on the decline in recent years, partly because of a substantial increase in the amount of teaching done by contingent faculty (adjuncts, full time instructors who aren't on a tenure track, and graduate student TA's.)  
The number of PhD graduates in mathematics has been fairly steady at around 1200 per year.  Most of these new graduates want to ultimately obtain a tenured faculty position, but there are only about 700-800 positions such positions available every year.  

There's a clear inbalance here between the supply of new PhD's in mathematics and the number of desirable faculty positions that they can chase.  

A good source of statistics on this is the annual survey done by the American Mathematical Society.