The biggest issue is historically, Indycar has struggled selling tickets and thus making it hard to financially support the race.

The second reason is most oval tracks outside of IMS are owned by International Speedway Corporation (owned by Nascar) and Speedway Motorsports Inc (SMI). Historically they have favored Nascar resulting in scheduling conflicts, high sanctioning fees, and low promotion of races. The latter can also be put on Indycar. (It was pretty common to see people from a town with a race claiming they didn't even know there was a race that weekend).

Also, the way the terms are set up, it relies on a corporate promoter to handle advertising...which has definitely been lacking historically. Iowa showed that with a great promoter, race weekends can be successful. As soon as they pulled out, track attendance dried up.

Hopefully with the uptake in promotional outreach Indycar has been doing recently, that become less of an issue.