Probably, the main reason was the temporal isolation of poncirus from other citrus species (the so-called allochronic isolation). The flowering of poncirus occurs about a month earlier. Although there should have been overlaps from time to time, especially with remontant varieties. There must have been some other human factors.
Under wet and hot conditions poncirus is making several flushes and flower up to three times a season.
Which suggests it should have been possible at some point to make crosses, especially with other repeat blooming citrus like lemons.
And yet, before around 1900, no crosses seem to have been made.
The likely explanation is that what few crosses did happen were mostly accidental and ignored or thrown away.
There was very, very little intentional breeding work with controlled crosses in the past. And even less long term needing work with multiple generations. Taking two very different pants, each with desired traits, and intentionally crossing them and breeding and selecting successive generations over long periods of time until both traits were combined was almost unheard of until the modern period. People saved seed, planted what they needed, and propagated stuff that they liked. On the random chance open pollination gave them sometime good on the first generation they're keep it, otherwise they just kept propagating the same stuff. That's fine if you have a few hundred years to wait for marginal improvements, but there's no way in a thousand years you'd breed a hardy citrus in a traditional agriculture setting. Landraces with open pollination and other old school systems have their advantages, but breeding new and interesting things from wide crosses isn't realistic under those conditions. The few trifoliate hybrids that did occur naturally in the past were probably tossed on the rubbish heap after their first fruit ripened.
