Calculators are becoming increasingly niche. A calculator is often artificially limited so that it's power in assessment is well understood. For example a calculator capable of symbolic algebra damages the ease of testing someone's algebraic talent. If you need more power than a calculator offers, laptops and smart phones reign supreme.
From a different angle, wolfram alpha is inherently not portable. It is a server based service. Creating a calculator using it amounts to creating a mobile data uplink, such as your phone. This has no benefits for you: it would need a network to provide 'data'!
You could of course create a portable Mathematica device, perhaps based around a raspberry pi. I struggle to imagine scenarios where the portability advantage over a netbook merits the effort.