There are several key differences between Nepal and Bhutan that contribute to why some travelers might return to Nepal more frequently than Bhutan:1. Accessibility and Cost:
Nepal:

Nepal is generally more accessible, with more frequent and varied flight options.
Travel in Nepal tends to be more budget-friendly, catering to a wider range of travelers.1
Independent travel is widely practiced.

Bhutan:

Bhutan has a "high-value, low-volume" tourism policy, which means travel there is generally more expensive.2
While the tourism policy has been altered, and independent travel is now more possible than in the past, organized tours are still very common.
Flights to Paro are limited, and the overall cost of travel is higher.

2. Variety of Experiences:
Nepal:

Nepal offers a vast range of experiences, from trekking in the Himalayas to exploring ancient cities and wildlife reserves.3
The diversity of landscapes and cultural experiences attracts repeat visitors.4
Nepal is very popular with trekkers, who often return to experience different regions and trails.5

Bhutan:

Bhutan focuses on cultural immersion and pristine natural beauty.6
While the experiences are deeply enriching, they may be less varied compared to Nepal.

3. Travel Style:
Nepal:

Nepal caters to a wider range of travel styles, including budget backpacking and independent travel.7
This flexibility appeals to many travelers who prefer to explore at their own pace.

Bhutan:

Bhutan's tourism policy has historically emphasized organized tours, which can be less appealing to independent travelers.8 Although as stated before, this is changing.

In essence:
Nepal's greater accessibility, affordability, and diverse range of experiences often lead to repeat visits.
Bhutan's unique focus on cultural preservation and controlled tourism, while offering a deeply rewarding experience, may result in travelers feeling they have experienced the country's essence in a single, albeit memorable, trip.