Q:

Why didn't Lady Olenna send for Daenerys?

Lady Olenna knew that Daenerys had dragons. Then why didn't Highgarden send a raven for help to Daenerys? Surely there would have been enough time for Daenerys to come to Lady Olenna's aid by mounting her dragons.
The only reason I could think of was that they did not want Daenerys to be a part of the battle at such an early stage. But was this reason worth losing a Westerosi ally?

A:

Lady Tyrell and Highgarden were caught off-guard, and when you include that one of Tyrell's most prolific bannermen, Tarly, defected to keeping their oath to the Queen rather than House Tyrell, they didn't stand much of a chance.
Earlier in either this episode or the previous one Daenerys even discusses mounting her dragons and attacking with just the three of them and her confidants tell her that's not a great idea.  So just Danny responding probably wouldn't have happened in any case.
Also, remember that Danny's main fleet was occupied by taking Casterly Rock, and subsequently heavily damaged if not destroyed by Euron Greyjoy, so responding by sea wouldn't exactly have been a choice either.

A:

By the time Olenna found out she (Highgarden) was under attack, they were already at her gates, and it was far too late to send word.

OLENNA: And you took your army, your real army and went where they [Daenerys' Unsullied] weren't.
JAIME: As Robb Stark did to me at Whispering Wood. There are always lessons in failures.

Jaime's entire strategy was a surprise attack he'd orchestrated, after learning the move from Robb Stark.

