India lost because in second innings they relied only on KL Rahul and Pant batting, if remaining batsmen have scored some runs then target would be high for England. The English team got good start in second innings and rest of batting take over responsibility once the opening pair got out. It's a high scoring game as total 1673 runs were scored in both innings which is highest match aggregate for England vs India in Tests. It's a five match series and English camp will be happy to start the series by winning the test.
In my opinion, the Indian batsmen did not play that badly in the first innings and even tried to do well in the second innings, but apart from KL Rahul and Pant, no one else's batting performance could produce any expected scores.
And yes, the early dismissal of their lower order batsmen in the second innings was one reason behind the Indian team's loss, but what I think is a bigger reason is that their bowlers did not do very well, along with the drop catches.
In both innings, the Indian lower order performed badly even though the top order made multiple centuries. In a test match where the batting pitch is good, you need to make a good score which is nearly impossible for the last batting team to chase and try to take their wickets before the day finishes or if that is not possible, take as much time so the game goes for a draw. But the score England got was an attainable one. Even though the socreboard shows not much difference on the runs they made, England has a much better batting compared to India in second innings.