Why was D-Day successful?

Transcript
War Diary of a unit in the 8th Canadian Brigade

Catalogue ref: WO 179/2866

6[June] Weather---Fine Enemy Air Activity---NIL.

At Sea

0630
D Day. Sighted BERNIERES and ST AUBIN. The landmarks which had been carefully memorised from air photos were clearly recognisable - the jetty at the harbour of COURSEULLES - the flat expanse of marsh land to the EAST - then the church steeple amid a profusion of trees, which marked BERNIERES and on down the coast to the cluster of b[ui]ld[in]gs surrounding the next church which marked ST AUBIN SUR MER. As the coast drew nearer the wind increased and a heavy sea made it necessary to consider whether or not the D[uplex]D[rive] t[an]ks [amphibious tanks launched from landing craft] were to be launched or should the t[roo]ps go in without their immediate assistance.

0700
Visibility lessening and the decision was made not to launch the DD tks. The expectant and eager tps of the assault coys [ie. companies] loaded into their L[anding]C[raft]A[ssault]s

0705-0735
At this time a sig[nal] was received that the A[rmoured]V[ehicle]R[oyal]E[ngineer]s were late. H h[ou]r for J2 [second assault group of Force J] was set back an additional 10 min[ute]s. To give the leading inf the s[up]p[ort] at the correct time, orders were issued to delay fire from the S[elf]P[ropelled] art[iller]y to conserve am[munitio]n for the assault. The armada slowly approached the shore and at 0725 L[anding]C[raft]G[un]s opened fire on the beach def[ence]s followed by the SP arty who opened fire after a slight delay had occurred in transmission of orders to all craft. The orders were to fire until 0807.