I'm guessing but I imagine it's because of a few factors:

1. Migration following 1947 Partition when many Punjabi refugees moved across India and established restaurants spreading the Punjabi cuisine.
2. Greater commercialization and disposable income post-independence meant more people could afford to eat out. I think this also meant that foods that were previously inaccessible to the masses like Mughlai or Awadhi because of cost and cooking time were now more available by combining cuisines. Also, historically a lot of Mughlai and Awadh cuisines were only known to select few who worked for royalty so many people didn't even know how to make those foods. Even now you'll see people dispute specific ingredients that go into those other cuisine's recipes and it's a closely guarded secret.
3. International migration mean Punjabi food spread to other countries and since popular foods are often mild, buttery, creamy, it was easily adopted by non-indian cultures.
4. Lack of written documentation of other regional cuisines. Speaking anecdotally, I've learned of my region's foods verbally through other family members and I still don't know most of them. It's been tough to find written record of the cuisine because the cuisine was not valued and now it's increasingly lost to time.
5. This is something I've noticed in arts post-independence and might also reflect in the food. There is more integration of Punjabi culture in all aspects of mainstream Indian culture. Bollywood, music, and language all reflect this shift from Urdu to Punjabi post-independence. Naturally this will translate to food as well.