I suspect they want better condition handling. Because JavaScript lacks a useful conditional expression beyond ternary expressions, condition handling in JSX generally uses boolean operators or ternary expressions. Suppose the conditional logic is in any way complex. In that case, your choice is to use nested ternary expressions, which can be hard to read or to somehow break the condition out of the JSX using a helper function or separate component so you can use if statements in a regular function context. SolidJS added conditional helpers to its JSX flavour to address this issue[1]. The React team seems ideologically opposed to the idea as it presumably moves JSX away from using only JavaScript for handling logic.
[1]https://www.solidjs.com/docs/latest/api#show