Simply put, because the Kings XI Punjab have a lack of death-bowling options. The player with the most experience and credentials for bowling at the death is Chris Jordan, but they've found it difficult to fit him into the XI. Mujeeb Ur Rahman has also had some success at the death, but he has not been in the XI either.
Moreover, the Kings XI opted to bowl out Sheldon Cottrell by the 13th over. While Cottrell had a good day with 1 for 20 in four overs, it meant the last seven overs would have to be shared between Mohammed Shami, James Neesham, Ravi Bishnoi and Gowtham. None of those options have been very good at the death. There was a case for slipping in the Gowtham over early on, but KL Rahul perhaps did not want to bowl the offspinner with Rohit Sharma set at the crease. He didn't go to Glenn Maxwell either.
That miscalculation would go on to hurt the Kings XI. In the history of the IPL, only 18 times has a right arm offspinner bowled the final over in the first innings. And the last time it happened before this game was in 2014. Gowtham ended up delivering the second-most expensive final over by a spinner, with Kieron Pollard and Hardik Pandya smashing 25 runs off it.
How do Kings XI solve their death-bowling woes? It looks increasingly likely that they'll have to bring in at least one of Mujeeb or Jordan, most likely in place of Neesham. How much that fixes their issue remains to be seen.
Does the ICC need to rethink the DRS rule on runs taken?
At the end of the 17th over when Mumbai were batting, Mohammed Shami appealed successfully for an lbw against Pollard. Pollard reviewed it and replays showed the batsman had got an inside edge on it, so the decision was overturned. However, in accordance with the ICC's rules, the ball was deemed dead and so the single the batsmen had completed did not count. It was a legitimate run for Pollard and Mumbai, and on another day, it could have significantly impacted the result.
Pollard and #MI denied a single. LBW given. Reviewed. Inside edge confirmed. But the easy run they took won't count. Dear @ICC, this might cost someone the World Cup someday. Need a rethink. Umpires will have to reserve their decision till the ball is dead. #MIvKXIP
Essentially, according to the rules, "the batting side, while benefiting from the reversal of the dismissal, shall not benefit from any runs that may subsequently have accrued from the delivery had the on-field umpire originally made a Not-out decision, other than any No-ball penalty". Also, "if an original decision of Not out is changed to Out, the ball shall retrospectively be deemed to have become dead from the moment of the dismissal event. All subsequent events, including any runs scored, shall be ignored."
Think back to the IPL 2019 final. Lasith Malinga got Shardul Thakur lbw with the final ball. Chennai Super Kings reviewed. Even if the decision was over-turned, Super Kings would have been denied the runs. Thakur and non-striker Ravindra Jadeja could have run two but their team would have lost by one run still because the runs wouldn't have counted.
In their last game against the Rajasthan Royals, Rahul seemed to consciously opt for the anchoring role while opening partner Mayank Agarwal went at the bowling. The merits of whether that should be the approach adopted or not are a debate for another day. But in a chase on a big ground against a quality bowling side like Mumbai, the Kings XI might have been better served by Rahul going harder at the start than he did, instead of slipping into strike-rotation mode.
He eventually finished with just 17 off 19, which is always the danger in a T20 when you play an anchoring role: you can end up dismissed before you have "caught up" so to speak, which leaves your side in deeper trouble.
The approach seemed more inexplicable given how the Kings XI had structured their side: lots of batting depth and fewer bowling options. They had Gowtham - T20 strike rate of 162.24 - batting at No. 8. In theory, that much depth should free up the openers to go harder.
He came into IPL 2020 on the back of some great form for Australia against England, but so far in the tournament, Maxwell hasn't really taken off. He had a good opportunity to correct that today, having walked in in the ninth over and with a free-striking Nicholas Pooran for company.
However, Maxwell couldn't get any sort of timing, power, or balance in his shot-making. He was particularly tied down by Rahul Chahar's legspin. It isn't a mode of bowling that has particularly troubled Maxwell overall - he averages 21.26 at a strike rate of 167.01 against leggies overall - but Chahar's execution was spot on. He bowled 10 balls to Maxwell, giving him nothing straight or straying on the pads, and slowing it up. If Maxwell wanted to hit him, he had to manufacture his own pace while going against the turn and reaching for the ball: nine balls were outside off, only one was on the stumps. The ploy worked, and Maxwell eventually fell to Chahar, slogging to deep midwicket.
Nair's game is suited to batting in the top order. He has not really played as a finisher in any IPL team or for his state side Karnataka, so if he's in the XI, he fits in better at the top of the order. In some ways, once they decided that Nair is part of their starting side, Kings XI were a little hamstrung in terms of batting order flexibility.
There is also the argument to be made that the likes of Pooran and Maxwell should get the maximum number of balls to face, which is best served by having them at three and four. If Kings XI want to split them to add greater heft to their batting, they would still make a better fit at three and five.
Nair hadn't come out to bat against the Rajasthan Royals, when the opening partnership went deep, so his role seems to be a floating one, where he comes in if an early wicket falls. What the Kings XI need to decide is whether they need a floater if they bat so deep.