why is that
You'll have to increase your receiving capacity. As far as I've understood this happens so the node that you're connected be able to know how many bitcoins there are left to be received (from your channel). In order to increase your receiving capacity click "Swap" and it'll show you some input fields. You can "convert" your sending capacity to receiving capacity.
If I've made a technical mistake in the second sentence above, I'd like to get a proper answer from a LN expert.
This will obviously cost you money in terms of paying a TX fee and the fee (if any) of the person who is willing to convert LN to on-chain balance.
If you can, it would probably be better to spend some coin on your existing channels if you are using electrum.