I think because of Supergirl moving from CBS to the CW they just want her in the crossovers for this year and next year they'll most likely bring the rest of team Supergirl.

Too many characters.
Clark's destiny =
Superman
,
LL
&
LL
.

Mon El is useless as super heroes go.

If Superman and Supergirl got mind controlled they'd be screwed. Miss Martian and Martian Manhunter would probably be immune with their telepathic powers.

From a tactical standpoint it would have made sense to ask for more help from her Earth. However I'm quite content it turned out this way. Its refreshing to see Kara operate on her own without her support team there. This should be about her spreading her wings and interacting with the other heroes instead of the ones she's used to.
I know we're not perfect, but the safest hands are still our own. - Captain America

but the others would probably help if she asked them
name the movie or tv quote
SQUEAL! Squeal like a little piggy poo (Note: not Deliverance)

Because Supergirl owes Barry a favor , and Barry is calling it in
Barry said
Remember last year , i helped you out and you promise you do the same
Nobody on Supergirl's world had promise to help Barry except Supergirl.

The brought in all of team Flash (in this episode at least), they'll bring in all of team Arrow and team Legends in the next couple of eps, but not all of team Supergirl?
That seems wrong to me
name the movie or tv quote
SQUEAL! Squeal like a little piggy poo (Note: not Deliverance)

so.. more people to mind control? more people to abduct?
the crossover so far does a decent job of juggling all these characters. any more and that balance breaks.
i kinda hope a future crossover forces felicity, cisco and winn to work with eachother, though. that could be funny to watch. 3 uber nerds finishing eachothers thoughts and such.

Totally. They could have used more backup from Supergirl's Earth - at least the following:
Alex, Winn, Manhunter, Mon-El, even Guardian.