Launch in US, page-6

From their website:

Splitit authorizes the full amount of the purchase on the shopper’s existing credit card and holds their credit line for the entire amount

I think they've made themselves too niche - note that Afterpay does 90% of transactions through customer debit cards, most Afterpay customers don't even have a credit card
I can see the benefit to Splitit - minimises risk to almost nil as they pre-auth the full amount on the customer's credit card and keep the remaining amount owing on hold until paid off. Benefit for customer is not having to pay interest, can split out over max 12 months.

Sezzle is an exact replica of Afterpay as far as I can tell - 4 instalments over 4 fortnights, no fees, can use debit or credit card. They have 537 merchants on board. Even their late fees are similar - for each payment USD$10 late once and USD$5 late second time - up to $22 AUD late fee per payment, $88 total (higher than APT's max $68).

Both companies have a much lower presence on social media than Afterpay.