Why didn't they let Mon Mothma show up in ST?

[Slowpokeking]

I mean her OT actress is still alive today and her PT actress reprised the role in RO. She certainly is a very important figure and deserved to be shown more than she did on screen. It was a shame that RotS cut most of her scenes. Why didn't Disney let the character show up in the ST as the head of the New Republic?

[SuperiorIronman]

As far as the lore goes she stepped down from politics.

As for where she could've been in the trilogy.., I don't know. Mon Mothma doesn't have anything really to do on Jakku and if you don't see her anywhere in the Hosnian system you can save her for a later appearance. That all being said TLJ is right after TFA which doesn't really leave room for an appearance. She could've been among the fleet at Exegol but her being seen isn't required. It also opens the door for showing this in a spin off.

[Frontier]

Things probably would've been easier for the Republic and the Resistance if she was still around.

I think Leia alluded to the Senate going downhill after she stepped down in Bloodline.

[ChrisIII]

Thing about the OT Rebel leadership, Admiral Ackbar in the sequel trilogy actually lived a few years longer than his EU counterpart. (The ANH and Rogue One version-apart from Mothma and Ackbar, didn't, as explained in the "Hope Dies" storyline; in the original EU Dodonna and others were captured but not killed).

BTW I'm pretty sure O' Reilly has been confirmed to appear in the Cassian Andor series once it starts filming. As she's leading the NR during the Mandalorian time period (and her home planet is the capital) I wouldn't rule out an appearance there as well.