SDK stands for Software Development Kit.
Solidity is a programming language. A constrained one at that. It can not do everything, and in the grand scheme of things it is relatively simple.
Solidity source code is compiled to Bytecode which can be executed by the Ethereum Virtual Machine.
An SDK is essentially a simplified interface to more complex code. Someone could make a Solidity SDK if they so wished, but given that Solidity is not that complex there is no clear benefit.
To answer your question, the EVM is limited in what it can 'execute'. Compiling an existing language with extensive features into bytecode that can be executed by such a simple machine is impossible.