Trails don't grow on trees. You have to pay people to make them...
Also lots of shows are low priorities in marketing terms - so you concentrate your marketing fire power on things that you want people to know are definitely new, worth watching etc.
Many of the trails are 'overproduced', now, I'm not advocating a return to the 70s and 80s practice of simply showing a random unedited 30 second extract from a programme, but some seem to be very expensively produced, and almost more elaborate than the programmes they are promoting. Also, they seem to concentrate on the most popular shows anyway, EastEnders, The Voice, Bake Off, or are they top rated because they are promoted ad infinitum ? Perhaps a little less overproduced, would allow for a greater range ?
I see elsewhere, that John McEnroe, commented on a trail stuffed into today's final, saying our TV was getting a bit too much like the US ?