Advice on publishing original theorems with easy proofs
There's a good chance the rejection is because your results are not interesting enough for the journal. In other words, the journal is looking for not only new and interesting results, but also complicated proofs and/or new techniques.
So: submit to another, probably less prestigious journal. Alternatively, you can submit to a similarly-ranked journal and hope the peer review process results in acceptance, which is entirely possible, because this particular reject reason is a judgement call and different people will come to different recommendations.