Unfortunately this trend of going to daven without a hat started about 15 years ago. It is a slippery slope where now it is “normal” to daven without a jacket as well. Recently people started coming to shul in shorts and/or sandals without socks (as if going to the beach!!). This is not yet as common, but it will be. It already started. Let me remind you that 15 years ago when people started coming to shul without a hat, they argued that it would never extend to go without a jacket! As I said its a slippery slope.
I won’t put the blame on any higher ups in the spiritual hierarchy (although I can). I believe it started when young gentlemen who considered themselves talmidei chachamim, would ask “show me where it says that you have to wear a hat”. They were not concerned about a mesorah or with doing the “right” thing, even though it may not be written anywhere. I am sure that none of their Rebbeim or Rosh Yeshivas davened without a hat or jacket, and nor did they while they were still in yeshiva. Its the kovod that should be shown in a shul and when davening.
Many will say that the hat and jacket, in todays world, is not necessarily considered as “kovod” anymore. Besides the answer of the slippery slope mentioned above, and besides the fact that we can’t determine what is kovod by measuring it against the decadence of the non-Jewish world, the fact is that this argument is just not true. The Courts, many theaters, and numerous other venues all demand a dress code and will not permit entrance without the proper attire. My gosh(!), even the Belmont Stakes which is a stupid horse race, in a smelly venue, and where everyone guzzles beer, has a dress code where the proper attire for men REQUIRES A JACKET AND A TIE. Don’t we owe the kedusha of a shul at least that??