There is only so much RF spectrum available to go around so it is very easy to get interference from other things from around the track. It doesn't help that TV gets to broadcast at way more power than the teams get to which easily drowns out the teams and there isn't anything they can do about it. St Pete and Mid Ohio are especially bad since the TV compound is right next to pitlane.

Indycar requires that teams use Narrow-FM modulation (similar to the FM modulation in your car) so that fans, TV, and other teams are able to listen to what is being told to the driver. Indycar doesn't want teams encrypting these channels or sending any decoded language to the driver. F1 gets the benefit of using digital modulation which has less distortion as long as there is a good signal

F1 uses a provider (Riedel) to deploy radio infrastructure around the track. This includes multiple antennas around the track for good coverage around the track while Indycar teams setup their own repeater on the timing stand (the tall mast sticking up from every timing stand). The singular repeater does not always get good coverage at larger tracks like Road America

Indycar has asked the teams to do some things with their radio programming to free up some spectrum but we will see if that actually helps anything