I am trying to display Rich Snippets for my store which is working fine for products and categories but it does not display rating/reviews for my website/homepage.
Below is my code
<span itemprop="reviewRating" itemscope itemtype="http://schema.org/Rating">
<meta itemprop="ratingValue" content="4" />
<meta itemprop="bestRating" content="5" />
</span>
Is it something I am doing wrong or Google does not allow this for homepage?
No, review rich snippets should only be used in cases where "the main topic of the page needs to be about a specific product or service." -- using them on your homepage in general wouldn't be like that.
your rating/review data does not contain all the elements needed fot the ratings to be shown. For example, there is a highest possible rating value bestRating of 5, but what is the lowest possible value?
What Product or Service are you actually rating (or reviewing)? Unless your microdata is very clearly linked to it then it won't understand - you probably want them to rate a http://schema.org/Service you provide, or to rate your business with http://schema.org/LocalBusiness or http://schema.org/Organization - so nest it inside another schema. The property aggregateRating is available within these, which you could use, semantically it means you are rating the Place ie the business at that location. I can't see any reason why this would not work.
What google actually says is on https://developers.google.com/structured-data/rich-snippets/reviews?hl=en&rd=1
We support reviews and ratings for a wide range of schema.org types, including businesses, products, and different creative works such as books or movies. When Google finds valid reviews or ratings markup, we may show a rich snippet that includes stars and other summary info from reviews or ratings.
Later on it is clear you can rate a LocalBusiness:
Make sure the review or ratings markup refers clearly to a specific product or service as shown in the examples above. Do this by nesting the review or ratings within the markup of another schema.org type — such as schema.org/Book or schema.org/LocalBusiness — or by using that schema.org typed element as a value for the itemReviewed property.
As per the Google's guidelines, Rich Snippet is not for home page. Here's a response from Google's Webmaster Trend analyst John Mueller in this reference:
No, review rich snippets should only be used in cases where "the main topic of the page needs to be about a specific product or service." -- using them on your homepage in general wouldn't be like that.
https://webmasters.stackexchange.com/questions/69292/rich-snippets-review-rating-on-homepage/69408#69408
