It's indeed because the rise in wealth isn't a product of Conservative policy, it's a product of globalism and foreign asset ownership. The majority of the richest people in the UK nowadays are not even UK citizens and/or don't even live here.So how can you say that the growth in billionaires in the UK is the product of Conservative Party policies? Similar increases are likely to have been observed in other economies as well.
The 'old money' of privilege and UK based assets at the top-end have been replaced by the 'new money' of pan-international businesses and individuals who have built up or acquired UK assets spread amongst their many other assets held abroad.
Look at James Dyson as a prime example. Conservative policy has no relevance to the fact that he is worth £10bn because he sells hoovers all over the world. If a UK government attacked Dyson financially then he just will cut away from the UK and move to a jurisdiction elsewhere, he wouldn't lose a penny in terms of net worth.
We can talk of other countries too and capitalism etc but the most billionaires in the world right now are found in China! A communist country!