It has been that way for awhile. I believe the 90s is the last time there have been more than 3 manufactures in IndyCar. There is rumor there may be third when IndyCar goes to a new engine formula in 2028. (Currently the IndyCar rules are for a 2.2L twin turbo, and they are going to a 2.4L formula in 2028.) It's just an economic decision as to whether it is worth the cost to develop and support given the number of customers you can spread the cost across.

For comparison, NASCAR's top class currently has 3 engine manufactures (Toyota, Chevy, Ford) and I believe F1 has 5 if memory serves me correctly.