What is the reasoning behind 1 ISA subscription per type per tax year

I get that, in each tax year, you can pay-in up to your ISA limit (at the moment £20000) into any number of – or create new – ISA accounts, so long as it’s only one of each type. So you can create a Cash ISA and a Stocks & Shares ISA, and pay into a Lifetime ISA all in the same year, as long as the total contribution does not exceed £20000. What I’d like to understand is, what’s the rational behind only allowing contributions for one of each type per tax year?

As an example, say I hold two Stocks & Shares ISAs. One is managed by me, and I decide what stocks go in, and the other is managed by a bank (i.e. a Managed Fund). Even if I were to stay within the limit, I can’t put money into both in the same year, even though that obviously has benefits wrt to risk management.

I really can’t think of a reason why this restriction is in place, and it would make more sense if the restriction was only paying into one of any type during one year. Can anyone explain why this is the way it is?