To increase fairness and reduce bureaucracy for both HMRC and employers.
National Insurance revenue goes into the general exchequer and is no longer ear-marked for pensions and benefits.
NI is an administrative burden on companies and the self-employed.
NI is subject to political manipulation (e.g. a recent promise not to raise taxes followed by an NI rate increase).
NI contributions are payable month by month so if your income is uneven, you may end up paying more than someone with the same annual income and this cannot be claimed back.
NI is unevenly applied with the poor paying disproportionately more than the wealthy (as it has a top end cap).
No government would ever dare introduce such a ridiculous, costly and unfair tax as National Insurance has slowly evolved into.
So why not simply abolish it ? increasing personal income tax by an appropriate amount to roughly balance the lost employees contributions and .increasing corporation tax by an appropriate amount to roughly balance the lost employers contributions. Pension entitlement could be tied to income tax instead of NI payments.