Because in delta connections you have only one level of voltage i.e. phase to phase voltage but in star connection you have two options of voltage level i.e. phase to neutral and phase to phase voltage. Definitely phase to neutral voltage is less than phase to phase voltage. Hope it will help you.
The phases are defined as voltage referred to ground (earth).
Start connection is "wiring to ground", delta connection means "wiring between phases". Within the 3 phase system voltage between any 2 phases is √3 * voltage vs. earth. (√3 results from the difference between 2 phases 120 degrees apart.)
cf. de.wikipedia.org - Sternschaltung resp. Dreieckschaltung
Unfortunately these pages are only available in German and Polish - or at least I was unable to locate an equivalent on the English pages :(