Main reason for that is narrow span of concentration - modern players can't keep focus for long and make at least one fatal mistake within a 29 overs (one session) span. Besides, because of playing too many white ball games, the hunger for big innings isn't there any more - say a 24 balls 59 is match winning in T20, a 54 balls 82 can be match winning in ODI; in that environment, how many would bother for a 319 of 575 balls in 10 hours?
Take Bradman, 52 Tests, 80 innings, 10 NO, 6996 runs at 99.94 - even that guy had 7 ducks (1 out of 11 outings, remarkably high ratio) and 23 innings in single digits. He had 29 hundreds & 12 50s, means around half of his career innings (39 of 80), he failed to cross 50, and yet managed a career average twice of 50. That's because, once set he was an immovable object - he had 12 doubles including 2 triples and at least another 12 over 150 ... once set, his innings were like 334, 304, 299* 254, 270, 244, 234, 187*, 170 .......
This is something I wrote long back (If Babar can end up with an average of 50+) that he won't neither Kohli - because Babar even at this age doesn't have a double and Kohli probably 1 or 2 - unless, you have few 200+ NO innings, it is really difficult to manage an average over 45-46. Also, modern players less value stats, particularly Test stats and average, hence don't put that much value to a Test wicket (not that much desperate to remain NO) - for example, in last Pindi Test, Mominul had a easy chance to remain NO on like 58 - he played a shot that in that situation, no Test player would have played even in 1980s, probably not even 90s. Graeme Hick was given till Tea to score his Hundred (SCG 1994) and when Captain (Athers?) called them in on Tea, Hick was 98* - his last 35 balls got him 23, and he was happy to be NO on 98, rather than get out trying to force a hundred.
50 years back, Viv had a similar attitude of modern day players - instead of hanging around for NO or converting into a mammoth innings, guy would blast his way with tremendous innings of 70-80, may be 110, that too targeting the main bowlers .... hence ended up with at average of just 50.23. At one point, till his 30s, Viv had an average of almost 60 (@ 75+SR), with 13/14 hundreds in like 50 Tests (It's like 1 in every <6 innings) and he had like 3 doubles - with age his eye sight faded away, but he didn't compromise his attacking instincts, hence his SR remained same, but for his last 70 or so Tests, he averaged something like 42.