Why did Mother Teresa use the phrase "it is a poverty"?

I should've checked the OED first.

4. Deficiency in an appropriate or desired quality; inferiority, paltriness, meanness; = poorness n. 3. Formerly also as a count noun.

So it's the same usage as in poverty of imagination (which is something of a journalistic stock phrase, if not rising quite to the level of idiom), or (to give a silly example) I have a poverty of self-control, so I ate all the cookies. "Poverty" in this sense without "of" immediately following is archaic, but not unprecedented:

▸a1387 J. Trevisa tr. R. Higden Polychron. (St. John's Cambr.) (1865) I. 11 (MED), I knewe myn owne pouert, and schamede..after so noble spekers..to putte forþ my bareyn speche.

a1425 (▸c1395) Bible (Wycliffite, L.V.) (Royal) (1850) Prov. vi. 32 He that is avouter schal leese his soule for the pouert [a1382 E.V. miseise; L. inopiam] of herte.

c1450 (▸c1400) Bk. Vices & Virtues (Huntington) 130 (MED), Þe first degree of mekenesse is for to knowe his pouertes and his defautes.

1597 Bacon Ess. v. f. 23v, By imputing to all excellencie in compositions a kind of pouertie or..a casualty or ieopardy.