i just wonder why this part of original implementation never became bigger?
Because of
Occam's razor.
it has no mining power scaling showing for the user using it and it is just used for making transactions itself basically
Exactly as intended, here is another Satoshi's quote:
Simplified Payment Verification is for lightweight client-only users who only do transactions and don't generate and don't participate in the node network. They wouldn't need to download blocks, just the hash chain, which is currently about 2MB and very quick to verify (less than a second to verify the whole chain). If the network becomes very large, like over 100,000 nodes, this is what we'll use to allow common users to do transactions without being full blown nodes. At that stage, most users should start running client-only software and only the specialist server farms keep running full network nodes, kind of like how the usenet network has consolidated.
Also, for that reason, you don't have built-in miner in the Core client, you have only some bitcoin-cli commands like "generateblock", but they are needed mostly on regtest, and not intended for mainnet use.
i wonder where the problem is for that?
You should make a separate application, that will talk with the Core client, that will be easier. If you try to integrate everything into the official client, you will see, why it is a bad idea. But if you want to try, then download the most recent version from GitHub, and explore it by yourself, if you are not convinced. In the worst case, you will end up with your own altcoin, in the best case, it will evolve into an alternative, minority client, that will be fully compatible with the official one. By walking this path, I hope you will end up with some separate application, but well, it will be your choice.
so why not giving a bit more space for more data for newers times build functionalities?
Ordinals blocked any block size increases above 4 MB. They set that limit in stone, if anything, it could be lowered, but not raised. As a recent example of that is OP_RETURN pull request, that was blocked as controversial:
https://github.com/bitcoin/bitcoin/pull/28130