Because mo úll is wrong. When mo or do come before a word with a vowel (or f+vowel), they drop the "o" and are written with the word. mo úll because m'úll. mo fear becomes m'fhear.
When 'do' contracts it can become d' or t'
'd'úll' or 't'úll' for 'your apple'.
t'úll is also correct as it has been said and written by native speakers for four hundred years. Writers like Peadar Ua Laoghaire and Mairtín Ó Cadhain wrote t'úll.
Oh d'úll is correct too, they both are. An seabhac, Tomás Ó Criomhthain would have also used t'úll, as did Séamas Ó Grianna. Many of the great modern authors did.
There isn't a "h" before "úll", except after "a" when it means "her" ("a úll" - "his apple", "a húll" - "her apple").
(There are a few other grammatical circumstances that trigger a h-prefix, but "mo" isn't one of them).